I attended the Apple World-Wide Developer's Conference last week (in additional to visiting people at Stanford, Google and PARC). Most of the actual content that was presented at the conference is covered by non-disclosure agreements, but a few generic observations can be made.
iPhone development is going to be huge. A diverse range of topics was covered, and the iPhone development sessions, in particular, were jam-packed and in several cases mandated the use of overflow rooms (and the main rooms were already very large).
WWDC was very large and was very well executed and professional. I have been attending various professional conferences for 20 years, and this was probably the most impressive I have seen in terms of organization, preparation and logistics. Of course, the fact that it was held in San Francisco didn't hurt either!
Since the iPhone development environment is very similar to the one for regular OS X, one can expect a lot of crossover from developers who originally have plans for only one of the two platforms. The UIKit for the iPhone isn't quite the same as the OS X environment, but additional the development effort required to port an iPhone application to the Mac might be a small as a day or two. This is also an important benefit for Apple.



