I started to do some work with this. I bought the extruder control board from makerbot and did some reading about hooking it up to my mill.
I got turned off after looking at all the parts beng made- they are really rather crude. Its neat to be able to make something this way, But Id like to be able to print parts that could become patterns for casting and everything Ive seen is much to coarse to be usable.
There was one of the thingvers units running at Cabin fever and I got to handle some of the parts it made and they were not very impressive.
I think if you make the extruded filament fine enough to make smooth parts it will require days of run time to build up anything interesting.
I may still move ahead on this, but for now Im kind of put off by it.