It depends on the application somewhat, but if you are serious about a startup it makes the most sense to start with the iPhone. The frameworks allow for the most "wow" factor with products, and there is simply a huge lead in number of units, and number of users used to running many different applications.
You may also want to consider other platforms (my vote for second to go after would be Android, and then Palm in third although that depends heavily on what your application is).
But something to consider is, you may want to start by doing one platform really well and if your application idea is well received, branch out. It's a lot of effort to develop for multiple platforms and each platform has various unique features you want to spend time taking full advantage of. I would also advise against using any of the cross-platform frameworks for the same reason, because when you target all you cannot really target one.
