Good to see that this is being blogged about. There's been a lot of research done to find the most efficient button sizes for handheld touchscreens. I think the next big push is going to involve design guidelines for truly mobile apps, apps you use while actually navigating the world and not stopping to work.

See http://www.mediateam.oulu.fi/publications/pdf/1076.pdf for a paper called "Target size study for one-handed thumb use on small touchscreen devices". It's a good starting point.

I had to think about this hard when we created Mobiphos, http://nirmalpatel.com/mobiphos.pdf to study mobile collocated photo sharing.