Just found these install instructions for Ruby 1.9.2-p0 and Rails 3 which don’t assume you already have a Ruby installation. The instructions are actually for Ubuntu 10.4 but I’ve had no problems installing on Ubuntu 10.10.
I was going to add step by step instructions and include any pitfalls I found but I didn’t so I’m not.
** DISCLAIMER ** These instructions worked for me on a completely fresh install of Ubuntu Maverick (10.1). I can’t guarantee they’ll work for you.