My Summary:
If you have the 'Brakes Hot' ECAM then Airbus are saying it is possible that when you retract the gear the brakes may be hot enough to ignite hydraulic fluid. Consequently if this ECAM occurs when wheels are up, the ECAM instructs you to put the wheels down.
If you have brake fans on, they cool the temperature sensors down more effectively than they do the actual brake discs. Airbus have calculated that 150 degrees indicated with fans on means that brake temps are equivalent to 300 degrees indicated without fans on.
Hence if you have been using fans, and the indicated brake temp is greater than 150 degrees, when you switch the fans off the indicated brake temp will gradually return to greater than 300 degrees. So if you have been using fans and the temp is indicating greater than 150 degrees and you then switch off the fans and takeoff, you may well get a 'Brakes Hot' ECAM when airborne, asking you to put the gear down again. This is the Airbus' only defence against wheel well fires as there is no fire detection system in the gear bays.
'Brakes Hot' ECAM has nothing to do with RTO's and brake energy, it is only there to indicate the potential for a wheel well fire. Indeed as the previous contributor states RTO trials are not carried out with brake temps indicating 300 degrees, but typically much less, in the region of 150 degrees.