Apple's latest Beta of iPhone firmware 2.1 reportedly disables any previous implementations of Apple's Push Notification System.
Beta 4 of iPhone firmware version 2.1 comes with the usual "bug fixes" description but completely removes any support for Apple's Push Notification Service which will help apps communicate via the Internet in the background without the app running.
Apple does offer an explanation for the removal of PNS stating that "further development" is required, but offers no timeframe on when it will return.
iPhone firmware 2.1 is expected in mid-September.