The Sigma DP1 is in stock and available from Amazon for $400. This is the price for today only, and only while supplies last, so move fast if you've been waiting for a great price on this camera.
Link: Sigma DP1 for $400 at Amazon
[Thanks Will P]
4/10/09 Update: The deal is now over. Today the low price on Amazon is $499, with All About Electronics as the seller. I don't know anything about All About Electronics and cannot recommend them. The current price with Amazon as the seller is $665.81.