iPhone 3.1 beta 2 now lets developers connect their phones to your development machine over Wi-Fi to run Instruments on; useful if you need to use your dock connector on the phone to connect to something else.
Other than this, there are a number of fixes that you can head over to the developer site to check out.
UPDATE: Reports are coming in that, predictably, some of the simpler tethering tricks no longer work.
https://gizmodo.com/how-to-easily-enable-tethering-and-mms-on-iphone-3-0-5295053
UPDATE 2: A few more features have come to light, including new accessibility options (VoiceOver and high contrast mode can be toggled with three clicks on the home button), as well as new video saving and editing buttons. [iPhone Dev]
