OK, so this sort of thing IS coming out:

Review: Cell phone GPS works well (CNN)

So it's just taking awhile for them to develop applications.

And as I think about it more, some of the things that take a long time for a handheld GPS can be done quicker on a cell phone- you start with a known cell, so that limits your location to begin with. It can also download the satellite data from the cell system, which is much quicker than directly from the satellites. Or so I would guess. I don't know if that's really how it works, but that's how I'd design it.