I know it doesn't matter, but I have so many things I wish my next phone/PDA could do, I'm going to list them for my own amusement. Even though most of them won't happen very soon, at least while the iPhone remains closed to native third-party applications.
I could manage data with specialized tools. Special-purpose editors for blogs like this, wikis for knowledge management at work, and genealogy records could take advantage of the multitouch UI.
[I happen to believe we'll see aGPS or PSAP in a very early release, based on Jobs's unusual and irrational enthusiasm about the satellite view in Google Maps.]
I could clean up and manipulate photos before sharing them online.
I could control my jukebox iMac from the couch without interrupting what's currently playing to use Front Row. And control the rest of the home theater, lighting, etc.
I could update Last.fm on the fly.
Maybe I underestimate what will be possible with Safari alone, especially if it includes Flash. It's already amazing how many of my "desktop" tools actually run in Safari or largely render via WebKit. If the iPhone is half as successful as it's poised to be, it will quickly vault Safari to the ranks of browsers you cannot afford to ignore. I'd be surprised if Microsoft isn't already talking to Apple about WPF/E.