Previewed at WWDC, launching in the fall.
iPhone 2.1 Beta 4 Seeded, Removes Push Notification Service
Interestingly, Apple has pulled the push notification service in this release "for further development." The capability was announced at Apple's Worldwide Developer's Conference Keynote in response to requests for background process support for third party iPhone applications. Applications that deal with messaging (such as AIM or Facebook) would likely stand to gain the most from the SDK enhancement. Apple has promised a September delivery of the functionality.
Apple began providing developmental support for the push notification service in iPhone Firmware 2.1 Beta 1. Not all registered iPhone developers have access to the seeds; the first seed was only available to iPhone developers who had been accepted into the program prior to July 11th (~4000 individuals).