HI Del,
I'm giving trade secrets away here, but yes we run SARK on mini-ITX all the time (among other boards). You have to make sure that your mini-ITX board is running true i686. A lot of the fanless VIA boards don't. They run something between i586 and i686 and are useless for our versions of asterisk, (Bristuff). The "Nehemiah" and "Luke" chips seem to be OK. We've had good results with VIA PD10000 (Nehemiah) and EK10000 (Luke) boards. We tend to run them with 512M memory and either a 2Gb or 4Gb SanDisk Extreme III CFCard instead of a hard drive. If you want to use a conventional hard drive then that's fine too.
Cases; - there're lots to choose from. Just Google for mini-itx case and you'll find lots. They very hugely in price and quality but the higher end stuff from companies like Travla is pretty impressive. We've had good results with the Travla C137 and C159. They aren't cheap but they are extremely well made.
Best
S