Here's a very neat prototype pen running on the MSM8660 development platform:

It's an ultrasonic pen:

http://www.engadget.com/2011/06/03/q...t-computex-vi/

It uses sound to determine its position accurately on a surface calibrated to 3 sources of sound emanating from the device. It implies that the device requires 3 speakers, putting it out of range of many devices, though the tech is still quite nifty.

I wouldn't mind a low profile device that could clip on to the top of a tablet and provide positional data to a pen and input directly into a device. There are already devices like this for paper, and it would be nice for them to have screen-writing support for android devices.