Hmmm... interesting challenge. Since the RepRap is an Open Hardware platform, there is a feasablility to this idea.

You would have to select your metal ahead of time, and build the printer to the material.
- The metal should be easy to get in wire form (your filament).
- I'd replace the hobbed bolt, in the extruder, with rubbered feed wheels.
- You would probably want to built a ceramic hotend and nozzle.
- Heat control would be critical. You want it just hot enough that the metal starts to ooze, not fully liquified.
- Ceramic print surface.

Lots of other considerations... still, it would be interesting to see someone try this.