Warning, rank speculation below
So on another thread on the tesla motors forums a while back, Franz was overheard in the design center talking to a guest while passing through the common areas. He said "...and the new seats will be perforated and ventilated..." as he walked into and out of the area. This is paraphrasing from memory from another person hearing a fragment of conversation that was taken without context... So take it for what it's worth, which is not much. I think it'd be funny if Franz was trolling us personally. Can imagine him in back trying to come up with some overheard fragment that would set the forums on fire.
But based on that overheard fragment and other things rumored that I expected by end of year, I delayed my production, so it worked out far as I'm concerned.
But taking it at face value, given the date this was overheard, I have to wonder if they had hoped and expected for the new seats to be ventilated, but it didn't work out due to whatever reason. I certainly didn't see any evidence the new seats were capable of ventilation. Maybe the other performance aspects of the seat weren't as good or the rest of the engineering just wasn't ready in time. If the later, could see them becoming an option later in 2015 as the X ships. It clearly is on their list of wants, just has to slot in with everything else.
Given that the seats are only available on the P85D right now and that only that model is shipping by end of year, I really feel like this was a push to get the top end out, vs. all supply issues worked out and everything exactly the way they wanted it. I expect the new seats will be available as an option for all models in a few months, and will probably have more options at that point, especially if they actually replace the current seats vs. being a paid upgrade in the ordering process. I could see ventilated being an option in the March or after timeframe at the earliest (or whenever the D orders taken now get done with), but if it doesn't make it by the time the X is starting to roll off in quantity, then I'd expect it to not happen till end of year or 2016.