Actually guys, this is maybe not such a good idea. I've just done a test heating the bed from around 22 deg C ambient to 65degC and measuring both the bed (inside the aluminium) and the surface temperature (thermocouple stuck to top of glass with Kapton tape. Spreadsheet is here if anyone is interested https://drive.google.com/file/d/0B_MwtHtQR_ZvU3dIYjRGZjVITkk/view?usp=sharing. Basically, at least for my particular bed, there is hardly any difference in the measured temperatures. Which only goes to prove that I shouldn't believe what a lot of people say about using glass on top of aluminium. I guess measuring the surface temperature might be worth doing if the print cooling fan had an effect but you'd need to have a very fast response heater to compensate.
Oh well, it was just a thought.