Quick note: If you wonder whether you can run/test iPhone 2.x apps on a device upgraded to the current 3.0 beta, I’d say you can.
I’ve upgraded one of our 2G models from 2.0 to 3.0 beta. Rebuilding our projects with the updated SDK and Xcode with a “Device 2.2 – Release” configuration worked like a charm, including the debugger functionality.
What we did notice though is that lots of apps we’ve downloaded from the App Store do crash on 3.0 beta. Seems as if they need to be re-built.
Be aware though: While it is possible to compile 2.2 applications with the new version of Xcode that gets installed with 3.0, Apple has specifically requested that developers not use that version of Xcode for compiling apps to go on the App Store.
Looking for what you’ve experienced in the comments.