Lion seems to be a memory hog (at this point) from what I've seen. During the course of my workday I was regularly finding myself using over the 4GB of RAM my 2010 MBP came with. I was getting several Page Outs in Activity Monitor. I decided to upgrade to 8GB of RAM and everything is running much more soothly than before. It almost seems like a new machine. I'm guessing that further point releases might change Lion's appetite for RAM.