If you try run Ruby Gem's command to update itself ($ sudo gem update --system) on Ubuntu you get a pretty stern warning that it's not recommended, and you might have issues blah blah blah.
There's a bunch of gems which complain if your gem version is out of date (and the default version with Ubuntu is), and I don't know when the official package will be updated.
Just so I know for next time, the command to force it to upgrade (at your own risk) is:
$ sudo REALLY_GEM_UPDATE_SYSTEM=1 gem update --system
I haven't had any problems from this on 11.04, but as always YMMV.