Here's an idea for a potentially awesome shield; I myself wouldn't buy it, but I bet a ton of others would:
The Lego NXT shield
Basically, it would be a shield that would allow you to easily interface your Arduino with your Lego NXT kit. It should, in some manner, allow you to easily interface to and control/read many more motors and sensors than what the standard NXT brick allows for (which is limited to 3 motors and 4 sensors). Even if it requires a separate breakout board to support enough of the special almost rj12 jacks, it would still be useful. I think, though, that you could fit several jacks on a shield.
Which could lead to another solution - if you made the shield oversized, you could add more jacks, plus you could add an LCD (serial based), and maybe buttons. Throw a library together for easy software development, plus holes on the upper shield sized to fit a standard hole spacing in Lego...
The only potential issue could be Lego themselves + but there are plenty of companies out there that sell sensors for Lego NXT, so maybe it won't be an issue (yeah right)...
