Here's an interesting post by a blogger named teucher that echoes something we've heard from developers before -- that the iPhone and the iPad will require completely different experiences. When the iPad was first announced, one of Apple's big selling points was that it already had a full library of software ready to go -- any iPhone app could and would easily run on the iPad. That's a huge advantage for any platform starting out. But as time gets closer to the actual device release date, it seems more and more like what works on the iPhone won't do for the iPad and vice versa. Take a look at this screenshot of IM+ for the iPad above -- while the UI elements are the same in general, you can't do a dual pane set of controls like that on the iPhone.
The big question is what this means for development -- originally, Apple sounded like they wanted to just have one App Store that you could install apps to any of your devices from. But since iPad and iPhone development are already diverging so much, it's very likely that we'll see an actual separation in the store. Additionally, as teucher suggests, Apple may need to make a separation between full and minor apps on the iPhone itself -- an app like "Clock" doesn't really call for the whole screen. That's where a widget system might come into play.
It'll be very interesting to see what happens with development and design divergence across the two devices going forward. April 3rd is almost upon us, and even the first few weeks of iPad apps should give us some indication of how the two platforms will relate to each other.