Diablo 3 is currently down in the USA while patch 1.0.3 - which adjusts loot drop rates at high difficulties and overhauls the cost of crafting - launches. It's due back up at 1pm PDT/4pm EDT.
The patch doesn't launch in Europe and Asia until tomorrow. The game will be down from 2am GMT/3am CEST on the 20th until noon GMT/1pm CEST - so if you've got urgent business with the Lords of Hell, get it all sorted out this evening. By 'get it all sorted out', I mean 'click on them until they die'. Check out our run-down for more on patch 1.0.3.