If you haven't upgraded to CS4, then you might not have seen how Adobe has consolidated and renamed various command dialog windows into panels. The Mask panel is one of them. There's not really any new functionality for making masks, but the new panel makes it easier to work efficiently with layer masks.
Bob Johnson hs posted an introduction to the Masks panel on his EarthBound Light site.
If you want a very brief overview, you'll probably like this tutorial. I would prefer a few more illustrations and a little more description. This one seems a little too brief.
Take a look, and you decide.