Rails - Jun 2010 - strange problem：no such file to load -- nokogiri on rails3

I am test the script named test.rb by command "ruby test.rb" and it
pass.
Then i converte it as rake task.
The rake task pass on rails 2.3.4 but fail on rails 3 beta4 with error
msg:
"no such file to load -- nokogiri"

===========================env ：rails 3 beta4, ruby 1.8.7, ubuntu
===========================test.rb
===========================require ''rubygems''
require ''nokogiri''
require ''open-uri''
require "iconv"

   url = "http://www.bugutang.com"
   doc = Nokogiri::HTML(open(url))
     doc.css("title").each do |c|
     puts Iconv.iconv("GBK//IGNORE", "UTF-8//IGNORE",
c.content)

    end
===========================claw.rake
===========================desc "Fetch product "
task :claw => :environment do
require ''rubygems''
require ''nokogiri''
require ''open-uri''
require "iconv"

   url = "http://www.bugutang.com"
   doc = Nokogiri::HTML(open(url))
     doc.css("title").each do |c|
     puts Iconv.iconv("GBK//IGNORE", "UTF-8//IGNORE",
c.content)

    end
end
===========================here is the environment :
RubyGems Environment:
  - RUBYGEMS VERSION: 1.3.7
  - RUBY VERSION: 1.8.7 (2008-08-11 patchlevel 72) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/bin/ruby1.8
  - EXECUTABLE DIRECTORY: /usr/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-linux
  - GEM PATHS:
     - /usr/lib/ruby/gems/1.8
     - /home/bugu/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/
*** LOCAL GEMS ***

nokogiri (1.4.2)
    Authors: Aaron Patterson, Mike Dalessio
    Rubyforge: http://rubyforge.org/projects/nokogiri
    Homepage: http://nokogiri.org
    Installed at: /usr/lib/ruby/gems/1.8

Is it one bug on Rails 3 beta ?
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