One thing I've been curious about for a while is what is the extent of the iPhone's ability to detect and track multiple touches. On the original iPhone, I've been able to detect and track 5 touches, though my fingers are large enough that I rarely could get it to recognize more than 4 distinct touches.
On the iPad, we've got a much bigger screen that will let all ten fingers touch at the same time without running together.
So, can the iPhone SDK track 10 fingers at once? Yep. It sure can. I wrote a simple program to just record the count of the touches set provided to the four UIResponder touch-related methods, and it had no problem identifying and tracking 10 separate fingers at the same time.
I don't know what the hardware or software limitations are on Apple's multi-touch technology, but it seems to be high enough not to be an issue.