To jump on the Wacom/N-trig bandwagon, I also think that the logic should be in the device.

In the 80s, Wacom developed a technology that is basically a grid of wires being a transceiver antenna, that does highly accurate positioning of an RFID-like induction coil (sometimes 2 or 3 in the same pen, one for each set of functions).

Thats it, the pen is basically RFID tags. No battery, no logic, no weight, the pen can be shaped nearly like anything (although early ones were a bit bulky).

N-trig developed their own variant of this technology, and Microsoft just recently bought them, to further integrate their technology into the Surface line.

The only problem I have with current Surface pens is they require an AAAA battery, instead of powering it like you would with Wacom's RFID-like signal.