*I know that this has been said a lot but: LiPo Sheild for the Arduino. Vregulator or something (having a high power on Vin would be nice too) with charging and protection.
I think this is a greatly requested item. Myself, I would go for it for some purposes even if it were 40 bucks or more. Forget the 'gas gauge', just a super rough reading (is it mostly full, or almost empty) coudl be very helpful.
Arduino stacking extenders: People with multiple bulky Protosheild creations will love you.
Another thing I think might sell well would be a kit to make a remote control camera. This would have 2 servos, a wireless camera, some kind of control unit likely involving the Joystick Sheild, and a pair of XBee or similar radios. And a mounting frame so the camera can pan and tilt. This would be good for telerobotics.
Come to think of that, a way to transmit video (even low res) over longer distances would probably be really good. There is a 40 mile XBee variant, isn't there? I don't even know if this is possible.
Also, what about some larger motors?
Now, here is one more idea. Something a bit like the Pololu Power Switch (including super low quiescent current), except that it has: 1, can be programmed (by serial, probably) to wake up after a specified time (should be able to be between about 10 seconds to several days or more, accuracy not super important) or -if this can even be done, which I doubt-wake on radio.