Even if it does go free after you purchase it, Apple is good about reimbursing people for stuff like that. Those that bought Macs 30 days prior to iLife '11 going on sale got a free download. I'm sure they would make the situation right.
I believe MobileMe is going to have two versions in the near future. A free version with basic features, and a premium version with more robust capabilities. If that happens, all those who've paid will be "fenced" in the premium version. The rest will be offered the basic MobileMe. I have no proof of this...complete speculation!
Just me $.02
Cosmo