http://blog.segment7.net/2011/02/19/rubygems-gem-activation-is-changing

We've changed the way gem activation works in RubyGems for the upcoming =
1.6 release (at the end of the month) for RubyGems 1.5 and earlier:

$ ruby -d -S thin start
[=85]
>> Using rails adapter
[=85]
Exception `Gem::LoadError' at /Library/Ruby/Site/1.8/rubygems.rb:274 - =
can't activate rack (~> 1.0.0, runtime) for ["actionpack-2.3.5", =
"rails-2.3.5"], already activated rack-1.2.1 for ["thin-1.2.5"]
Missing the Rails 2.3.5 gem. Please `gem install -v=3D2.3.5 rails`, =
update your RAILS_GEM_VERSION setting in config/environment.rb for the =
Rails version you do have installed, or comment out RAILS_GEM_VERSION to =
use the latest version installed.

With RubyGems from trunk:

$ ruby -I ~/Work/git/rubygems/lib -S thin start
>> Using rails adapter
>> Thin web server (v1.2.5 codename This Is Not A Web Server)
>> Maximum connections set to 1024
>> Listening on 0.0.0.0:3000, CTRL+C to stop

RubyGems now lazily activates gems so a gem's load paths will not be =
added to $LOAD_PATH until its files are needed.

We would like testers for our changes to gem activation. If you would =
like to help us test you will not be able to release gems. Gem authors =
should be prepared to downgrade to RubyGems 1.5.2 if you test the trunk =
version.

If you wish to test the new version of RubyGems you will need to check =
out the repository using git fromgit://github.com/rubygems/rubygems.git =
then run ruby setup.rb. You can also test without upgrading by usingruby =
-I path/to/rubygems/lib. If you're running a ruby executable such as =
thin run ruby -I path/to/rubygems/lib -S thin.

Please report any issues you have on the mailing list (subscribe here) =
or in the #rubygems channel on chat.freenode.net(check that drbrain is =
active).

When you're done testing you can downgrade to RubyGems 1.5.2 using the =
Downgrade Recipe in the UPGRADINGdocument.