iPhone Application Backward Compatibility With iPhone OS 3.0

According to Ralf Rottmann at 24100.net, you can keep developing your iPhone OS 2.x apps using the new SDK and run them either on an iPhone with 2.x firmware or one with 3.0 beta. Here's what Ralf did:

"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."

Other tips and comments from developers posting to 24100.net:

• Many apps downloaded from the App Store crash on 3.0 beta; probably need to be re-build
• Apple has specifically requested that developers not use the Xcode that gets installed with 3.0 for compiling apps to go on the App Store.
• If needed, You can install multiple versions of XCode by installing each SDK with a new location for the Developer Essentials. For more details, see the comments posted here.