I'm familiar with 3D printing, and the G-code concept. I'm also comfortable with programming.
Can anyone give me the G-code (or probably the M-code, actually) to read the bed temperature?
Is there an equivalent of
M105 (Get Extruder Temperature)?
M105 should give you the bed temperature.
For future reference you can find a general list of G/M codes here - http://reprap.org/wiki/G-code
Most firmware files include a list, Marlin has it in Marlin_Main. I have no idea how often the list is updated but they don't change often - https://github.com/MarlinFirmware/Marlin/blob/RC/Marlin/Marlin_main.cpp