Interesting idea, but most of the people use their smartphone with only one hand.
Yep that, and knuckle your screen everytime isn't going to do it any good unfortunatly, but why not digg the idea -)
I'm a former student of CTO, Chris Harrison (he was a TA for the programming usable interfaces course at the HCII). Queexo isn't only concerned with detecting knuckle touches, but all sorts of potential combinations of surface input touches, sensitivity, and dimensionality detection on a touch screen. I highly recommend checking out his site http://www.chrisharrison.net and his work with the Future Interfaces Group at Carnegie Mellon http://www.figlab.com .
Thanks for those details Jon ;)
These guys should be their first client.