This is absolutely the right direction, but not in the way you think. This shouldn't be branded as a phone -- size and look matter too much. It should be branded the tool of the future, designed for people people in the field: electricians / construction workers / scientists / tinkerers. Let me plug in an altimeter, leveler, or multimeter. That is what is missing on my iPhone, and this is the first thing I've seen modular enough to pull it off.

And, yes, I've been thinking Tricorder this whole time=).