I was revisiting a Rails 3 upgrade last night, rolled things back this morning, and realized Rails was requiring all the wrong gems. Turned out the problem was I was using the global RVM gemset for all the Rails 3 gems, which, as the name would suggest, is for gems that are used by all gemsets. Silly me. Solution:

rvm env # revealed the multiple gem paths
rvm use ruby-1.8.7@global
# http://geekystuff.net/2009/01/14/remove-all-ruby-gems/
gem list | cut -d" " -f1 | xargs gem uninstall -aIx
rvm gemset create rails3
rvm use ruby-1.8.7@rails3
git checkout rails3 # branch for the upgrade w/ Gemfile etc
git checkout master
rvm use ruby-1.8.7@rails2
# everything back to normal