Boxcar 2.0 is now waiting in Apple’s submission queue, and the author expects it to be live sometime between September 3 and September 7. What does boxcar do?
For Twitter, it can send you push notifications of anything you want — @replies, direct message, even for all of your tweets if you want (though that’d get a bit crazy for most people).
The new version also supports Facebook, allowing you to get notifications of all Facebook alerts. The Facebook 3.0 app won’t even have that, though it’s expected for Facebook 3.1 down the road.
Finally, it also has push notifications for email. You tell your email account to forward everything to them (for notification purposes only), and it’ll keep you informed.
All in all, this should be a pretty sweet app. It’s not free, but it’s reasonably cheap ($2.99). Should be well worth it for most folks. Look for it to come out in a few weeks.