I'm all for simple designs using low cost off the shelf products! Looks like an all-metal design which would make this very reliable and the accuracy would be reproducible every time you use it. One thing I'd watch out for: The belt might grind against the slot extrusion when it winds around the wheels and loops around the pulley. Since the tooth side is facing towards the extrusion your belts will wear out faster and result in motion stops. I have a similar design where the extrusion is placed above the motor for this very reason.
For a laser diode, I reccomend getttig one from DTR Laser Shop, his prices and quality can't be beat.
HPB, I am aware of the possibility. I did model this in Sketchup, and with the 30T sprocket and GT2 belt, it appears there will be adequate clearance to where the belt won't rub at all. Of course, I haven't actually built this to verify, but I intend to build one soon. http://www.openbuilds.com/members/anthony-bolgar.35746/, thanks for the tip. I'll have a look at their wares. MG