Installing Rails on Ubuntu

I’m posting this here because I was following the instructions on p36 of the pdf. This could just as well go on the Rails Talk Group or on a Ubuntu list.
I’m completely new to Linux, having suffered with Windows for too long. I’ve just installed Ubuntu Hardy Heron on my laptop and I was following the instructions on p36 (to the letter!).
All seemed to be going well until I tried reinstalling to Rails v2.02 and updating gem to 1.20. Since then, I’m getting an error –
/usr/bin/gem:23: uninitialized constant Gem::GemRunner (NameError)
I’ve tried purging and reinstalling to no avail.
Is this a problem in the book or have I messed up?

thanks
Martin

Apparently, there is an incompatibility between newer versions of gems and older versions of rails. This might help, as well as the railscore discussion thread it references.