Corebell

They have developed a mobile robot that is compatible with Lego NXT.
It includes dual motors and acts as a solid base to build your own mobile robot, which saves you having to build your own base from individual Lego pieces. You’re supposed to attach your own NXT Brick on top and plug it into the mobile base. Obviously it’s compatible with the Lego stud pattern and it has expansion ports for additional sensors.

Fig. 1:

Update: Since this time Corebell seems to have moved into other markets, like industrial robots. However, here’s a video that shows their NXT-compatible “Mobile Modular Robot”. Video: Robot Dreams