open hand and fist recognition?

trying to think of a method of detecting between an open hand v. a closed hand (fist) and i'm at a loss. My requirements are that it has to detect the bare hand (not a ring or led on the palm of the hand or anything like that), nothing can be attached to the user, should be arduino based (i don't want to have to deal with kinect and the instability of windows) and hopefully it'll be able to recognize more then one user. I did back the kickstarter cmucam5 and think that i may be able to differentiate the characteristics based on size. The issues that may arise is the fact that the amount of skin color detected will be different based on if the user has long or short sleeves on. I guess i could have an initial calibration at the beginning but if the user takes off a jacket, he'd have to recalibrate.

does anyone have any other ideas that could work?

Seems like a job for image processing (and I've seen markerless tracking solutions that seem to do things like this) but Arduino is not a suitable platform for doing that.

would capacitance or tempature change when you make a fist?

@scott_fx I am also looking for working solution.

If you have anything on your mind, please let me know.