They are backordered from all the usual suspects (Rainier Arms, 762SASS, DSG Arms, etc.) and Armalite is out-of-stock at the present to the best of my knowledge. Best to get on the "notify" list from one (or all) of the above retailers first.
Then, keep a watchful eye on the PX here, as well as the BARFCOM Equipment Exchange section dedicated to the AR-10/308 Cal ARs and snag one there first if one pops up. See here: http://www.ar15.com/forums/f_7/120_AR10.html