It makes nearly no sense to bring Sigma cameras out in anything but a Sigma mount. It's the razor/razorblade theory, and they make a good deal off of lenses. Assuming that this camera is up to snuff, it will demand some very good EX glass. And handholding will be difficult with this pixel pitch (I noticed an increase in motion blur moving from an SD10 to the SD14/15---and this camera is a 50% increase in the linear resolution). I've noticed that Sigma has really increased their OS lens lineup too. And the 8-16 mm wideangle is a stellar performer.
With regards to comparison, I'd love to see an end to it. I print 12 x 18" from the SD14/15 (and SD10 frankly), and have never had an issue with 'not enough' pixels. My pals with their APS-C Nikons and Canons love the details and sharpness of these. This camera will be like an 'apple to orange' comparison to the current crop of APS-C sensored cameras. It ought to be much, much better. If you have good glass on it. If you have it steady... etc.
It's really great to see Sigma make such a leap only two years (a little less) after acquiring Foveon. And if the new sensor is improved with regards to noise and exposure behavior, this'll be a very nice camera to own (price is rumored to be 'comparable' to a 7D, according to the US manager of Sigma). If they sell enough of these, I'd expect that we'd see a similar effort in a full frame or 1.2 crop camera down the line. And if they can jam this thing into their compact body (along with the dual True II processors), that'd be something to have in the pocket.