The way it was once explained to me, the publisher would make available an electronic version of the book/whatever to the various e-Book vendors, who would then do whatever voodoo they do to ready it for distribution in their particular proprietary format. Said vendors would do so either to meet hard street dates in the case of those select titles that come with fixed release dates, or at least so that the e-release is commensurate with the hardcopy starting to hit shelves. How that works/worked would vary from vendor to vendor (Mobipocket, Palm, Amazon, etc.).
I don't know if this is still how things are done, or if the publishers are readying releases in the various formats themselves. Looking at the S&S site would seem to imply that the publisher preps for distribution via a couple of the formats themselves (MS Reader, Adobe, Palm), but they may very well contract that work.
However it works, the process is still beyond the scope of anyone you're liable to find posting here. :/
