The cold snap we're experiencing in Britain at the moment has made me ponder this.  Answers from my friends and relations so far have included:

Because the water is moving (But how does this stop it from freezing?)

Because the water is under pressure

Because the temperature is above zero underground

Beacuse of the otters that the water companies keep swimming around in the pipes (I think this one was a joke)

So...why doesn't the water in the mains pipes freeze when it is below freezing point outside?

If you were to take a huge long tube with both ends covered over and stick one end up into space and leave the other one here, before uncovering both ends, would there be a huge vacuum effect? Assuming the tube is strong enough not to be crushed!