District Heating - I think not...
"Water-cooling is a better way to transfer heat from data centre kit - from thermal conduction chip modules to racks - than air cooling."
That's all fine and dandy at the level of the physics - indeed liquids are better than gases at transferring heat away as they have much more carrying capacity. Car engines are (almost) universally liquid-cooled and IBM themselves used to require their mainframes to by liquid cooled back in the days of ECL logic (and that was direct liquid cooling plumbed into the data centre, not a hybrid system via heat exchangers in the racks).
However, there are very good practical reasons why direct liquid cooling of computing equipment is a major league pain-in-the-neck in data centres. The plumbing types up lots of space, there is the ever-present danger of leaks, air-locks, installation costs and so on.
it should also be noted that there are very considerable problems with making use of waste heat in district heating systems. What comes out of a data centre is low-grade heat (whether direct, or via the air-conn). Technically it's a very low entropy energy source, which is just another way of saying there are vast volumes of warmed up air, not the concentrated supplies of hot water that you need for district heating schemes (where you really want the water approaching boling point). It's simply not practical to pipe thousands of cubic metres of warm air around houses.
There are ways of increasing the usefulness of waste heat by increasing the temperature of the exhausted heat from an air conn system. However, you don't get something for nothing - the downside of that is that the efficiency of the air conn system will plummet. It's a simple matter of thermodynaics - heat engines, carnot cycles and all that stuff. Essentially the data centre air-conn will have to use more energy to make something more useful for heating hot water and the like. There will be a net gain of course - there are heat pumps which make use of the ground being warming than air temperatures during the winter, but the hardware end expenditure would be enormous.
Some might argue for running liquid cooling straight into the computing hardwarfe and running the internals of a computer at 90 degrees or so (and you'd have to run the components as hot as possible to get a decent temperature gradient to make this liquid cooling work well). I would have severe doubts about this - you may just get away with it for the processor, but there are plenty of other components which won't run reliably at those sort of levels. I'm also not sure you want lots of near-boiling hot water flowing around your data centre.
All in all, if you want to blow the warm area from a data centre into a large industrial building opr a commercial greenhouse, that might work and be cost-effective. However, as a general district heating system - I think you can forget it. There's much more to be gained by reducing the power required by the electronic bits and pieces in the first place and reducing the need for expensive air-conditioning rather than vast, complex and very expensive schemes to turn the high-entropy waste heat from a data centre into something more useful.