You should see the furore over at the DPR forums... rats, sinking ships springs to mind!
My more sanguine view is that this is commercial bravery which may well make or break the Sigma SLR range and the SA mount as a result; their decent lens range will of course carry on for other manufacturers.
If the camera truly does match a medium format quality, then its target market is very small - because most who already shoot that stuff have a preferred system, and Sigma just doesn't have the pedigree of Hass, Pentax, Leaf no matter how hard they try.
If it doesn't, then the target market is even smaller - because nobody will spring for a camera twice the price of a 1DS, D3X or an M9 when all those cameras [Canon, Nikon, Leica] have long pedigrees, end user support, quality control and preferred uses/users out in the field.
Plus, the Sigma SA mount offers no unique advantage because all the very best Sigma lenses are available in EF or F mount aswell.
I like Sigma, always championing the underdog, but this makes a Leica S2 or a Pentax 645D look like a sensible decision. Time will tell - if the SD1 is £3000 street in 12 months time, then stocks aren't shifting.