I really like this design, much like the design on Dave's site buildyouridea.com. But the one on his site provides for adjustement on the bearings. I think that idea could be incorporated easily enough with this design. I love this design, seeing how you just need a CNC mill, and not a foundry setup. I think this will definetly be the ticket for my CNC router. Great job!
Edit: Also, would their be any disadvantage to 3 sets of bearings? Too much friction? I would think that three adjustable sets would be extremely ridgid! I concur with Joe though, I will have to switch it so the weight rides at 12 o'clock, and I'll have to have a supported rail. My gantry will be all steel and weigh quite a bit!