As of November 2011, should I buy the NXT 2.0 now, or wait and buy the NXT 3.0 later?
First off, there is not going to be an NXT 3.0. If there is, I would be extremely surprised.

This rumor was started back on April 1st, 2010 with a post on this blog (scroll to the bottom). Since then, many of people have been tricked. They even made another post the day afterward confessing that it was fake. Despite this, the rumor traveled to people who don't follow The NXT STEP, making its origins uncertain to them, and assumed it to have been announced by TLG.

With the Robotics Invention System, they stopped at the 2.0, and just waited some years before releasing the NeXT generation. I expect the same pattern here. So just go ahead and buy the NXT 2.0.

As for just Mindstorms set updates in general, there was once talk about expansion packs for the NXT kit that were supposed to be released by LEGO a few months later, but that discussion took place several years ago, so it is probably a no-go.