In NetBeans 6.1, I created a new project and the "Run Rake Tasks" menu was empty (it only said "Refresh List"). I found someone else through Google who had the same problem, but no solution other than "use NetBeans 6.5 beta".
After fooling around a bit, I simply un-installed Rails 2.1.1 and re-installed it and voila, the "Run Rake Tasks" menu was populated with all its tasks. Bizarre.
P.S. It wasn't completely random that I chose to re-install Rails 2.1.1. When I tried to check on the plug-ins I had installed, I got a message that said to install Rails 2.1.1. My gems list said it was already installed, so I decided to un-install it and then install.