While looking for a precise stylus I came across this one as well: http://www.cregle.com/ It looks like it solves a lot of the gripes with normal capacitive styluses, but I haven't tried it.
This is the same hardware as the XO stylus in a different case. Same OEM for both. They look great, the demos are wonderful, but user reviews say that they don't work well in the upper third of the screen and require frequent recalibration.
Interesting, I hadn't seen the XO before. It's too bad these ultrasonic styluses don't live up to the hype. I would love to take notes on a new iPad and ditch the paper notebooks completely. It'd be sweet if Apple shipped a solution, but I'm not getting my hopes up.
If you go to Cregle's iPen Kickstarter page - Comments section - you'll see it didn't live up to expectations. (Google iPen, should be first result).
Calibration issues, broken, etc. I was really looking forward to a precise stylus (pressure sensitivity is number 2 priority to me), and I'm still waiting…