Experimental base for possible robots
What you're seeing here are the not-yet-released, larger yellow hubs for use with the LEGO treads. They have some smaller black ones (smaller in diameter) but I found that the treads would tend to rub against the motors or jump the track. The larger yellow hubs allowed the treads (in my example) to completely avoid any parts of the NXT motors.
I tried various configurations of the hub-triangle (placing them closer or farther away with the top yellow hub of the pyramid being lower or higher). There are a lot of configurations that work, but I had the most fun with the one you're seeing pictured.
Next, I'm going to try and make some longer configurations so the thing can go over larger obstacles. The base you're looking tips over easily when it accelerates so I've placed the brick on top and further to the front to balance this - the torque of the motors makes it pop a wheelie if the brick is any further back.