No, CS3 should remain the same. The only things that may change are file associations (automatically open in CS4 instead of CS3), but you can change that back. LR will see CS4 as primary editor for as long as it's installed, but there's a workaround for that too.
I've looked over all of it. I was hoping that the graphics acceleration would spead up the brushes, but they seemed to actually get slower. I use those a lot, so that's not good for me.
Also, I think that I would have to update the whole Creative Suite, so I would end up paying $6'' just to update Photoshop. I use the other stuff, but not enough to worry about updating them right now.