Well, we’ve been waiting for this since day 1.  While I am particularly excited about the 3rd Party App SDK, the Exchange support is even more important.  Without that, there’s no corporate email (as such), and without corporate email, there’s no market outside of personal use.

Apple adds Exchange support to iPhone, unveils SDK

March 6, 2008 (Computerworld) Apple Inc. announced on Thursday that it has licensed protocols from rival Microsoft Corp. to add Exchange support to its iPhone.The company also unveiled a software developer’s kit (SDK) that hands over the tools third-party developers need to write native, rather than Web-based, applications for the smart phone.

However, both new functions — Exchange support and the ability to install non-Apple applications — won’t be added to the iPhone until June, CEO Steve Jobs said.