Upgrading to ruby-1.9.2-p290 on my development machines (Windows and Linux) was a breeze. There were some extra steps needed to upgrade on the server:
1. rvm get head
2. rvm reload
After that, just:
3. rvm install ruby-1.9.2-p290
4. rvm –default 1.9.2-p290
5. rvm gemset create 3.1.1
6. rvm gemset use 3.1.1 –default
7. gem install rails –no-rdoc –no-ri
and ready to roll.