This design is currently under development, first test of the heating concept has been done, but:
the whole bed has not been constructed and tested (by printing a mendel) yet! 
So please discuss, test, rinse repeat and take care when you do this at home... 

This bed uses circulating hot air from a hair dryer (fen) to heat a hollow polycarbonate bed - piece of multilayer fluted PC roofing. We are working with 4 layer 10mm Makrolon, but even 7 layer 16mm boards can be had. The fen we got is a 10 EUR Hyundai. (btw, next project might be 14EUR Hyundai kettle heated bed... ;)

The fan in the fen is turned on all the time. For this we disconnect it from the heater coil (acting as an ac voltage divider) and supply it with a steady 12V, possibly switched by the motherboard if you wish to turn it off when the printer is idle. Constant circulation of the hot air evenly disperses the temperature over the whole bed, much like a Mr.T glass oven.

The heater coil operates in pulse mode, its on time ratio controlled by the PID controller acting on temperature information from the thermistor. We had a cache of some cheap plastic linear thermistors. Nominally they are 19.5mV/C, we iced and cooked them and found them closer to 15mV/C. Rated to 125C, they will fight for proving the concept. Use a glass RepRap thermistor to be sure.

The lower layers of the PC build bed are sealed with high-temp (1500C heh) furnace caulk to prevent hot air blowing trough them. With this we turn them into isolation, so most heat escapes trough the upper surface, heating the build volume not the underside of the machine. There are also 300C caulks. (if you have no way to get something like that, plug each of the small holes with a small bunched up piece of paper.,, ;)

To get the air flowing, on both sides of the bed, where the holes are, the bed is sealed to a moderate-sized cavity. The cavities are connected by a silicone hose to the in and out "ports" of the hacked hair dryer. The temperature sensor is embedded under the bed surface, with measures taken not to block airflow trough any of the flutes. Additional sensors may be put in each cavity for more chance to follow and control how hot parts of the system get.

The hair dryer has a built-in overheat safety that will kick in if the temperature inside the heater coils get dangerous. As this is hopefully the hottest part of the thing, it should not by itself cause a fire so easily. Whether the built-in fan motor will live to tell about it is still unclear at this point.

The PC bed we have does not have a really flat surface - it has a slight relief in line with the structure. Also it may need some reinforcement to resist bending by the warping print stuck on it. We will only see how serious these problems are when we get to the stage of printing on it. Stay tuned...

This research is encouraged also by the ABS on Polycarbonate tests done by nophead.

Our RepRap meetings: Eksperimentalna delavnica 3D tiskanja.