Update: Tesla contacted me today indicating "the factory was not tooled to make grey seats after 12/31" and offered a free "upgrade" to the premium seats.
Personally, I liked the gray leather but alas..., anyways they offered WHITE / TAN / BLACK and since the S60 I ordered already carried the premium package, whichever option I chose would also be vented. Hooray, I guess.
tl;dr - grey seats have been discontinued as of 2017; if you had configured your 2016 (but-now-really-2017-not-yet-built) order with them you will need to choose a new WHITE/TAN/BLACK premium seat option.