This is a cool project using the NXT, lasers, and diffraction gratings.
What I did
1 I took some diffraction grating glasses and cut them up and built the machine shown here.

2 Shooting the laser through the gratings to get this effect.

3 Spin the motors in opposite directions and take a time exposure to achieve this effect.

Or this...

This is very cool and I urge you to try it if you can.

Just a small safety note:
When using lasers never shine it directy into someones eyes or a camera lens.
If you shine it in someones eyes there retina will be damaged in a matter of seconds
Please be responsible. Never shine it in someone's eyes.
Regards,
Ryan