I doubt this will ever happen. Look at some of the older dell USFF computers like the Optiplex SX260 and SX270 - They used laptop drives which only require a single connector running from the motherboard instead of an IDE cable + Power cable. If a giant like Dell never got them directly on the motherboard I'd think its pretty safe to say that we won't see this happen with the SSD drives.
To the above poster that was referring to ease of swapping for component manufacturers, the only companies that would have to worry about it are the biggies like Dell, HP, etc. They make more use of case design to allow for easy parts swaps than redesigning the motherboard.
If it ever did happen it would most likely be mounted on the edge of the mobo with a 90 degree plug so that the SSD sat parallel to the mobo instead of sticking out perpendicularly (like a video card does). This would allow for a ledge to be put on the mobo tray to keep the SSD from having to put all it's wait on the socket.