most of that "new business" on different carriers would be current iphone users fleeing like rats from a burning ATT ship. i dont think the shares would change much at all with all the choices available now, as well as ones that are on the way.
You have to think though, the iPhone alone is enough to get people to STAY on ATT's network despite how much they dislike it.
On top of that the iPhone is able to get people to switch carriers, this is a powerful indication of how well it would do if it was released into the wild.
If a single phone could propel ATT to this many subscribers, I wouldn't underestimate iPhone sales if it were on Verizon, Sprint, or Tmobile.
Currently the iPhone doesn't offer enough for me to switch, but IMO I think it has the most polished mobile OS. Everything on the 3GS is faster, smoother, and its physics engine when scrolling or panning is unrivaled. A snapdragon processor of today is needed to even keep up with a year old 3GS, that is saying something.
I'm looking forward to Froyo and its 400% speed boost, which hopefully fixes a lot of the choppiness that Android OS is experiencing.