Apple has recently released firmware 3.0 Beta 3, build 7A280f, to developers. Along with the firmware there are also updates to the SDK. Changes include performance improvements, user interface updates and alterations to Spotlight search. How does this affect you? Well one big issues with Beta 3 is it will not run apps that aren’t made for 3.0. This means none of your apps would work but the ones you’ve developed for 3.0 yourself. (as of writing this article, beta 3 is not jailbroken either.)
UPDATE: The note below is for the emulator in the SDK, not for the firmware on the iPhone. (Thanks to Tetanus in the comments for this info)
“Applications targeting devices prior to iPhone OS 3.0 beta cannot be tested in this seed, this bug is intended to be fixed in a future seed.”