The Sigma DP2 Quattro is now available for pre-order, although it is not yet known when it will begin to ship. The MSRP is $999, which is what I'd figured it would be (although with Sigma you never know).
The Quattro is a lot different than the Merrill cameras. Sigma claims that the DP2 Quattro has an "equivalent" 39 megapixels. I doubt that is true. The Merrill cameras have about 28 "equivalent" megapixels, and I'm not convinced that the Quattro cameras will actually produce more resolution than that. But it might. It's hard to know until people actually start using them.
It is also supposed to be faster, have better battery life, have better high-ISO performance, and fewer noise issues than the Merrill series. That's great if true. Someday I hope to get my hands on one to find out, but it won't be right away. Until then, I'll happily use my DP2 Merrill.
I suspect that pretty soon the price will drop on the DP Merrill series. In fact, prices have already dropped significantly. They'll likely continue to drop as more Quattro cameras are sold.
Which should you buy? I can't answer that for you, but a heavily discounted DP2 Merrill will be a better value than a full priced DP2 Quattro.