Apple has seeded some iPhone developers with a new beta iPhone firmware for testing. The new version is labeled iPhone OS 2.2 beta 1 and is accompanied by a new beta of the iPhone SDK (build 9M2611). Beyond "compatibility testing", there are no other differences noted by Apple. It's not clear yet if the new builds have any further implementation of background push notifications that have been promised by Apple.
The latest purported email
from Steve Jobs claims that this feature is "running a bit late":
We're running a bit late. We want to get it 100% right the first time.
the 2.1 Firmware to the public on September 12th. iPhone 2.1 addressed a number of bugs and stability issues with the iPhone and App Store.