iPhone OS 3.1. We'll let you know if we see anything crazy, like usable Gmail, true background applications, or a notification UI that doesn't make you want to throw the phone against a very hard surface at high velocity.
Update: Looks like Apple has closed the door on hacks that enable tethering in this release -- at least for AT&T according to early reports.
Update 2: We've just been informed that beta 2 also introduces what appears to be a public API for manipulating live video. Devs are still testing the capabilities but it could open the door for augmented reality applications like Layar to come to the iPhone 3GS.