Thats a really interesting question. I know that the forces involved in cutting any steel would be considerably (~20x?) higher than for routing wood & plastic. In one of the labs we did, we measured force for turning a large steel round on the lathe. Calibration wasn't precise by any means, but measured force was about 1000N = 100 kg = 220 lb.
I know Andy (Lionclaw) did some aluminum work, which seemed to work well. Like you said, you'd definitely need a different spindle setup to lower the speed and increase the torque, which would inevitably be heavier. With some modification and component repositioning (to balance the added weight balance) I think it would be do-able. The gantry sides would need to be stiffed to compensate for increased side loads when cutting the in Y direction. Gantry tipping may also become a problem when cutting in the X direction. Depending on how much you're taking of with each pass, you could reduce the loads. Coolant issues would definitely arise though since it is MDF (or ply or whatever). But I guess it could be tried...I'm not responsible when your machine soaks up the coolant and doubles in size like one of those 'Just add water and watch it grow!' toys. Without any coolant you would go through a lot of bits/cutters.
Check the result and a video of Andy's aluminum cutting here:
http://www.cnczone.com/forums/showth...=14331&page=18 (My latest project, Machine #2)
I'd like to think it was possible, but my instinct tells me it would be better to go with a different design and materials.