Have you noticed that it takes a while for iTunes or the iPhone's App Store to determine if there's an update available for your phone? That's because it looks at the list of software you currently have and then it goes and checks to see wether there's an update for any of your currently installed apps.
In order to implement a push system to notify you of updates available for your apps, Apple would need to keep track of all the installed apps on every single iPhone/iPod touch. Now, they surely know which software you have downloaded at some point, because it is associated to your own account. ***BUT***, considering that you may have apps on your phone that are associated with different iTunes accounts (not only yours), also taking into account that you may sync, install and uninstall an app anytime on and from your phone, there's no way for Apple to know what really is physically installed in your device or if you even keep copies of every single app you have downloaded in the past. You would end up receiving notifications for apps that are no longer on your iTunes or even apps that you keep in iTunes but are not installed in your phone. You would also receive notifications for apps that you probably deleted a long time ago.
So I think this explains why you don't (and wont) receive push notifications for app updates. Unless they change the way things work right now, I don't see a simple way for this to happen.