Apple will do research and see if there's a big enough group to justify a 64gb iPhone. While it is obvious from this thread a lot of people on here would like it, outside MacRumors that isn't so true. Most people that have an iPhone are very casual users(I mean like what, a third have never synced it before?) and I think having a third option at a higher price point for a 64 gb iPhone would not sell as well as the other two. If Apple thinks that making a 64gb iPhone will be financially beneficial, they will. If they think the 64 gb version won't sell well, they won't.
What I think they should do is drop the 16 gb version (keep the 3g at 8gb, and the 4 at 16gb) and make the next iPhone 32 and 64gb if they want a larger iPhone.