I\'m running Ruby 1.9.1p243 on CentOS, and I decided to install rvm to handle upgrading to 1.9.2 or downgrading to 1.8.7 (whichever turns out to work better for rails3).
Typically rvm support is easiest via IRC (#rvm on freenode) - in this particular case, what does running "type rvm | head -n1" show? it should show "rvm is a function". If not, that means the line to source rvm isn't being run correctly and hence switching doesn't work. Typically this means you either have a return in your ~/.bashrc or you missed adding the line to source rvm.