Generalizing Software Defined Radios, Software Defined Antennas, and Field Programmable Gate Arrays: Software Defined Electronics. A diverse set of electronic elements (resistors, capacitators, inductors, etc) of various specifications would made available for connection in any topology. Partial re-configuration would be supported (e.g. while the device is operating, one part of it could instruct the substrate to reconfigure another part).
This would solve the problems noted in http://en.wikipedia.org/wiki/Software-defined_radio in the "Ideal concept" and "Receiver architecture" sections, by allowing the system to dynamically reconfigure its analog signal conditioning prior to the A-D convertor step.
For even more fun, combine with RepRap? and microfluidics.