I don't know if my "problem" is related to what you are experiencing, but here goes.
Its not a huge deal, just odd. I can put my PB to sleep by closing the lid just fine. Last night, it was on my coffee table, lid closed, sleeping, and charging. Then I heard the hard drive spin up, CD-eject sound, and the sleep light turned off. So I just waited for a few seconds. About 15 seconds later, it went back to sleep. Strange.
I use "problem" in quotes b/c its not a big deal, it went back to sleep. But had it not, and it was in a bag or not plugged in, it would have drained some of the battery (not that much tho, since it is set to sleep after some time).
I have a bluetooth keyboard which can wake it, but it was across the room w/ no keys being hit. I don't believe I have the setting for allowing the network to wake the computer, and I have Airport anyway, so I think I also remember reading that the setting has no effect in this case.
Oh well, just a little mystery for me,
-Jake