gem location problem from rake task

I have installed the following gem which is in my environment.rb.
However when I try to run a rake task, it gives the error:

environment.rb
config.gem "devise_invitable", :version => '0.2.3'

$ rake mytasks:task
(in /home/allen/www/lines)
DEPRECATION WARNING: Rake tasks in vendor/plugins/paperclip/tasks are
deprecated. Use lib/tasks instead. (called from
/usr/lib/ruby/gems/1.8/gems/rails-2.3.8/lib/tasks/rails.rb:10)
Missing these required gems:
  devise_invitable = 0.2.3

You're running:
  ruby 1.8.7.249 at /usr/bin/ruby1.8
  rubygems 1.3.5 at /home/allen/.gem/ruby/1.8, /usr/lib/ruby/gems/1.8

Run `rake gems:install` to install the missing gems.

so I do "rake gems:install" and it installs.

$ gem env
RubyGems Environment:
  - RUBYGEMS VERSION: 1.3.7
  - RUBY VERSION: 1.8.7 (2010-04-19 patchlevel 253) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/local/bin/ruby
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-linux
  - GEM PATHS:
     - /usr/local/lib/ruby/gems/1.8
     - /home/allen/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/

In the directory:
$ cd /usr/local/lib/ruby/gems/1.8/gems/
allen@ubuntu:/usr/local/lib/ruby/gems/1.8/gems$ ll
total 132
.
.
drwxr-xr-x 6 root root 4096 2010-08-20 13:50 devise_invitable-0.2.3

HELP!

BTW when I do "gem list" the gem appears:

# gem list
allen@ubuntu:~/www/lines$ gem list

*** LOCAL GEMS ***

actionmailer (2.3.8)
actionpack (2.3.8)
activerecord (2.3.8)
activeresource (2.3.8)
activesupport (2.3.8)
authlogic (2.1.3)
bcrypt-ruby (2.1.2)
crack (0.1.6)
devise (1.0.8, 1.0.7)
devise_invitable (0.2.3) <-----------------
fastthread (1.0.7)
formtastic (0.9.10)