After much investigating tonight, I discovered that the reason my MB would never fully go to sleep was NNW (I currently have 3.2b23 running). I don't know why, but somehow it prevents the machine from going to sleep, and only the display does. Of course, if you manually tell it to sleep, it will, but it won't automatically, no matter what your Energy Saver preferences are.
Also: I tried completely uninstalling. Didn't make a difference.
Also: I tried completely uninstalling. Didn't make a difference.