"Push notification" is dev-speak for "I would like to run apps in the background, pretty please." At the moment, Apple does not permit apps to do different things at the same time, although the capability would be a tremendous boon to users. For example, an instant messaging app could signal the arrival of a message even though the app isn't running or the iPhone is asleep.
Apple incorporated push notification in the beta it released in August and devs expected to see the feature debut with the iPhone's 2.1 firmware update. Inexplicably, at least to outsiders, push notification went AWOL when no one was looking.
Blogstreet has been jammed with rumors for some time about why Apple yanked push notification service. One blog posited that it just wasn't ready for prime time. Another speculated that AT&T stands to lose money if users are able to use instant messaging free instead of AT&T's paid text messaging service. Yet another suggested Apple just couldn't handle the job, which would require its servers to ping iPhones when apps needed to update.
The latest rumor traveling around the block is that Apple plans to give users the option of running one or two background processes with the next firmware update and open the door even wider with the intro of a more capable, next-gen iPhone. That phone is due out in the middle of this year, according to those who have their eyes peeled on Apple.