Interestingly, mixed over/underground tunnel systems like the London Underground effectively pumps heat back underground (the trains heat up from both the sun and braking etc., and radiate that heat into the tunnels) to the point where the clay surrounding the London Underground tunnels is many degrees hotter now than when the London Underground opened. Cities with underground systems might effectively have a lot of excess heat available to pump out. There's some feasbility study being done on reusing waste heat from ventilation, but I don't know if there's been any work done on to what extent the London Underground heating of the ground in the city centre does to the efficiency of ground source heat pumps.