Sony isn't promising anything, they're not saying it's 100% but they aren't saying much else than that. Could be 99.9%, could be 75%, nobody knows right now so we can't really knock them for that yet...
If you've got 50 ps2 games, and you can keep playing them on your ps3, it's a nice feeling for a lot of people. Nobody likes buying a new system and only having 1-2 games for it anymore. If they buy a couple of launch games, and they're kind of sh!tty (gee whiz, imagine that...overpriced games and they're piss-poor), they can have something to fall back on without having to disconnect their new console and put the old one back, and without having to hope their old ps2 laser still works...
Most people who still play PSone games do it on their ps2, they don't have the playstation hooked up to the tv...Probably the only reason they pull out the PSone games from time to time, because it's not a terrible inconvenience to do so (get behind the tv, disconnect ps2, connect psone, undo it all when you're finished).
Same with xbox. Any 360 owner who wants to play halo 1 again is putting it into the 360 instead of hooking up the old xbox...unless it's on their xbox hard drive but that's a whole different ball of wax...