The only technical difficulty I could see with this would be in checking for minimum wall thickness on-the-fly. Otherwise, it might be possible to scale a model's size down below printability for a particular material, or oversize it to the point that minimum wall thickness needs to be increased.
Simply scaling is extremely easy, but checking for minimum tolerances per material might be more difficult.
Still, something I'm surprised hasn't already been implemented.
Last edited: Jan 31, 2012