Well, I'm pretty certian AT&T is the culprit behind MMS not working but you'd think Push would be an all Apple thing, right? I'll tell ya...up to this point, I was happier with my 2.2.1 jailbreak setup than this 3.0 joint. I was excited like a kid but so far, two of the three biggest reasons I was looking forward to 3.0, the other being C&P, aren't even there.
Push notification apps are just making it into the app store. Beejive had an update ready and waiting to be submitted once 3.0 was live. If the version you have doesn't have it, keep checking for updates, I'm sure it will be available soon.