Take the carpet to the Tate Modern and enter it for the Turner prize? ;-)
If you want a serious answer:
Mop up as much as you can with kitchen towels. If it's the type of carpet which doesn't mind a bit of water, get a big bucket & sponge and mop it over to try to get some more of the milk out. Then head to your nearest branch of Pets at Home to buy a bottle of this stuff:
http://www.simplesolution.com/products_uk/stai n&odor.htm
(It'll kill any stale milk smells).
Chris