There are two (that I'm aware of) low-cost rapid prototyping systems out there - Reprap and Fabathome. They are three-axis positioners with a head that instead of cutting or drawing or probing, successively builds up layers of glue or some other material to build an object in three dimensions. These systems also have software that figures out the positioning and extuding code based on 3D models. As far as I can see, their dedicated software controls the machines "on the fly" so it builds as it goes (i.e. no G-code type output for the machine to follow--they seem to have eliminated that step entirely).
The obvious question is - how hard would it be to engineer a retrofit for an existing three axis positioner? To slap an extruder head on it and call it a "fourth axis?" G-code has been around a long time and works pretty well so it would follow that with some additional logic, you could extrude material (frosting, glue, whatever) without a terrible amount of difficulty since all that engineering has been done up front (as in, timing on "when" and "how fast" to extrude versus "where" the head is at a given time). Then you'd have a multi-tasker instead of a dedicated "fabber" as they call them.
What are your thoughts on all this?