Wait till you have to keep up with the changes when 3.1 then 3.2 come out six months within each other, and start to change and deprecate things that you learned in 3.0.x
If you've a Rails 2.x app with plugin ActsLikeWhatever, and you run into some problem with that plugin, chances are the fix will only be in the newest, non-Rails2, release.
Like what happened to Rails 3.0.7 and Rake 2 weeks ago.
I wish the Rubygems guys would put down the crack pipe though. They're the one problem in this setup and, admittedly, they're a massive problem :(
Another thing to learn.