Apple has seeded some iPhone developers with a new beta iPhone firmware for testing. The new build is iPhone OS 2.2 beta 1. 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.
Steve
Apple released 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.