I am about to decide if clean install Snow Leopard or clean install just Leopard, to be honest. There is still an issue if we need to do clean installs, so to me that clearly proves there is an issue with the OS upgrade, how much proof do we need??
I believe a new battery is just an eventual fix, as the new battery will run down fast in the next few months, just the feeling I have, Snow Leopard consumes much more battery, and makes laptop hotter (probably just due to the higher consume), that combined with semi-wasted batteries (probably most of us have a year or year an a half laptops) may be the whole issue.
Random shutdowns I think can be due to the fact that swollen batteries (due to high temperatures or faster wasting) make bad connection, I can confirm mine looked a bit swollen just before got a replacement.
Jose.