Hi David, I've just popped the lid off a 790 and taken out the hard drive caddy. I have a mini-itx board turning up tomorrow so i can double check, but my first thoughts are that the mounting holes are in normal positions.
The front fan also runs independent of the front I/O port (which does not use a standard USB or audio connection).
That's the good news, the bad news is you won't be fitting this into a mini-itx case any time soon. It appears the be an extended mini-DTX board. it is the same width as m-DTX board, but is longer as well. The CPU socket is mounted a lot deeper into the case than most board, it is affectively where the DIMM slots are on most board.
You probably fit it into a m-atx case though.
I hope this helps. Personally I'd stick with the nice looking Dell or build a m-ITX solution from scratch.