Maybe 20GB is enough for 'hundreds of RAWs' if you have a 2MP camera and shoot exclusively at ISO100 ...
I'm exaggerating, of course. But the RAW files from my camera are huge - about 10-15MB each, IIRC ... I've got about 80GB in my active library, with all of that plus another 40-50GB backed up off-site. So MobileMe's 20GB is absolutely
worthless to me as a backup tool. But I did buy a Family Pack on June 10th, because I want push email/calendars/contacts for my two new iPhone 3Gs next week. And in the meantime, I'm finding the integration with iLife to be VERY cool.