$ ./script/server -b 127.0.0.1
=> Booting Mongrel (use 'script/server webrick' to force WEBrick)
=> Rails 2.1.0 application starting on http://127.0.0.1:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
** Starting Mongrel listening at 127.0.0.1:3000
** Starting Rails with development environment...
Exiting
/Users/pistolfly/railsprojects/test/app/controllers/application.rb:7: undefined method `init_gettext' for ApplicationController:Class (NoMethodError)
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:215:in `load_without_new_constant_marking'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:215:in `load_file'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:354:in `new_constants_in'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:214:in `load_file'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:95:in `require_or_load'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:60:in `depend_on'
	from /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:456:in `require_dependency'
	from /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:18:in `define_dispatcher_callbacks'
	 ... 40 levels...
	from /Library/Ruby/Gems/1.8/gems/rails-2.1.0/lib/commands/server.rb:39
	from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
	from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `require'
	from ./script/server:3

とりあえずエラーが発生しているApplicationController(application.rb)のinit_gettextをコメントアウトしてmongrelを起動して、ページにアクセスすると、今度は以下のエラー。

  Status: 500 Internal Server Error
  undefined method `file_exists?' for #
    /Library/Ruby/Gems/1.8/gems/gettext-1.91.0/lib/gettext/rails.rb:281:in `render_file'
    /Library/Ruby/Gems/1.8/gems/gettext-1.91.0/lib/gettext/rails.rb:279:in `each'
    /Library/Ruby/Gems/1.8/gems/gettext-1.91.0/lib/gettext/rails.rb:279:in `render_file'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/rescue.rb:181:in `rescue_action_locally'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/rescue.rb:125:in `rescue_action'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/rescue.rb:203:in `perform_action_without_caching'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/caching/sql_cache.rb:13:in `perform_action'
    /Library/Ruby/Gems/1.8/gems/activerecord-2.1.0/lib/active_record/connection_adapters/abstract/query_cache.rb:33:in `cache'
    /Library/Ruby/Gems/1.8/gems/activerecord-2.1.0/lib/active_record/query_cache.rb:8:in `cache'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/caching/sql_cache.rb:12:in `perform_action'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/base.rb:529:in `send'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/base.rb:529:in `process_without_filters'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/filters.rb:569:in `process_without_session_management_support'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/session_management.rb:130:in `process'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/base.rb:389:in `process'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:149:in `handle_request'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:107:in `dispatch'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:104:in `synchronize'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:104:in `dispatch'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:120:in `dispatch_cgi'
    /Library/Ruby/Gems/1.8/gems/actionpack-2.1.0/lib/action_controller/dispatcher.rb:35:in `dispatch'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/rails.rb:76:in `process'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/rails.rb:74:in `synchronize'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/rails.rb:74:in `process'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:159:in `process_client'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `each'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `process_client'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `initialize'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `new'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `initialize'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `new'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel/configurator.rb:282:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel/configurator.rb:281:in `each'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel/configurator.rb:281:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/bin/mongrel_rails:128:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel/command.rb:212:in `run'
    /Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/bin/mongrel_rails:281
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:502:in `load'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:502:in `load'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:354:in `new_constants_in'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:502:in `load'
    /Library/Ruby/Gems/1.8/gems/rails-2.1.0/lib/commands/servers/mongrel.rb:64
    /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
    /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `require'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:509:in `require'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:354:in `new_constants_in'
    /Library/Ruby/Gems/1.8/gems/activesupport-2.1.0/lib/active_support/dependencies.rb:509:in `require'
    /Library/Ruby/Gems/1.8/gems/rails-2.1.0/lib/commands/server.rb:39
    /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
    /Library/Ruby/Site/1.8/rubygems/custom_require.rb:27:in `require'
    ./script/server:3

対処方法は以下のページに。
rails2.0.2 → 2.1.0 に移行 - 夜の Discovery
Edge Rails and gettext: undefined method file_exists? (NoMethodError)
config/initializers/gettext.rbを作成する。
内容は以下。

require 'gettext/rails'
module ActionView
  class Base
    delegate :file_exists?, :to => :finder unless respond_to?(:file_exists?)
  end
end

config/initializers/gettext.rbにgettext/railsのrequireを書いたので、config/environment.rbから、gettext/railsの読み込みを削除。

# config/initializers/gettext.rbに移動
#require 'gettext/rails'

この問題は、Rails-2.0.xからRails-2.1.0でActionView::Base#file_exists? メソッドが ActionView::TemplateFinder#file_exists?に移ったことが原因らしい。
Ruby-GetText-1.92.0で対応済みなので、上記回避方法は不要となった。
Rails2.1以降では、environment.rbで、require 'gettext/rails'ではなく、config.gemを使う。

require 'gettext/rails'

ではなく、

Rails::Initializer.run do |config|
  :
  :
  config.gem "gettext", :lib => "gettext/rails"
end