I've got a MBP 3,1 with 4gigs of RAM. I've been wondering the same; I think technically, my machine will support it, but I don't know if the whole experience will be a good one. Lion currently runs well, a little slow, but well. I don't want to end up in a similar spot like I did with my iPhone 3G, which technically ran iOS 4, but was very slow doing it.