Yes, all new app store submissions must be 3.0 compatible. I have run across quite a few that just crash and burn (on 3.0 BETA 5) for me however, but that was pre 3.0 requirement.
Now this is just my personal experience, but I haven't had any trouble with apps running on 3.0 with only one exception (I have 63 apps ). A few were a bit flakey at first and then ceased to have problems, and even more were fine since day 1. The only thing I've had trouble with is the eBay app -- it hasn't allowed me to log in since around the time that either beta 4 or beta 5 was released. I open the app, try to sign in and it continuously looks like it's trying but nothing happens.
It's safe to say that most are already compatible with 3.0 and the few that aren't will either be fixed soon or removed from the store by Apple when the time comes.
Thanks. The reason I asked was I got an update for Wunderadio today labelled 3.0 compatible and wundered whether it was the first of many over the next few days.