Rails 3, Paperclip and checking out particular revisions

Hey, got a weird one. I've been playing with a couple of Rails 3 apps
and today I have broken both. Seemingly by updating my paperclip gem.

Gemfile
.....

gem 'paperclip', :git => 'git://github.com/thoughtbot/paperclip.git',
:branch => 'rails3'

It appears the latest version of paperclip or at least in the branch
has broken in Rails 3. So I've had to go into the bundled repo and
`git checkout <revision>` to get it back to a revision that actually
works. So does bundler support a particular revision from a git repo?

Cheers