I'm sure smaller scale data centre heat to district heating schemes must already be in place. Fundamentally you are using the same technology to cool the data centre (a heat pump), just pushing that heat into hot-water / steam, rather than dumping into the air.
It's a bit hard when the temperature deltas are so small, though. It probably works well if you're pre-heating air before the main heat source in a very cold region, but otherwise you'd need to use some sort of heat pump to get it to move around.
I'm sure smaller scale data centre heat to district heating schemes must already be in place. Fundamentally you are using the same technology to cool the data centre (a heat pump), just pushing that heat into hot-water / steam, rather than dumping into the air.