Ack! I just tried to upgrade rubygems... and it destroyed my working version of rails 2.3.5 (yes, we use it for a client that has not yet upgraded due to the quite reasonable "it ain't broke" assumption).
Now I can't run script/server without one of the following errors:
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/gem_dependency.rb:268:in `==': undefined method `name' for "Ascii85":String (NoMethodError)
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/gem_dependency.rb:119:in `requirement': undefined local variable or method `version_requirements' for #
Chris Oliver provides a quick fix for getting back to a previously-working version in his description of the undefined local variable or method `version_requirements` and there's an active bug-report for the undefined method `name' for "Ascii85":String
For now, though, it looks like the only solution is to downgrade and hope for a fix... which seems to be coming only for edge rails. I really don't like it that I have to maintain and old version of rubygems just so that I can run my client's perfectly functional rails stack. :(