Now despite it being only 'coming soon', our good friend Xander over at BotBench has managed to get his hands on one.
The mailman brought me a cool new toy to play with this afternoon! It’s the Mindsensors GlideWheel-M! These are so cool, you can now use the Large XL Power Functions motor with the same degree of accuracy as an NXT motor. The GlideWheel-M is just a single unit thick and fits very nicely onto the motor. You can mount it in any orientation you like. It’s really easy to use; if you know how to use an NXT motor, you can use this.
Check out his blog post for a very nice video as well.