Hope it isn't early 2013 but late 2012
There is good chance that it will be 2013. I don't think Apple is going to bite on the version 1.0 "ac" chipsets. They'll likely wait for "1.1" or "2.0" something where the initial implementers take the arrows in the back and Apple can roll out something that is extremely solid.
The Airport routers have a very good reputation of "just working, no drama". Typically that comes from actually testing the product in a variety of environments and send/reciever pairings before deploying it.
Beside the Extreme was just updated in 2011. Updates have been along the lines of :
2007 (two: early and late )
2009 (two: early and late)
2011 (didn't have to come back with a tweak this time.)
http://en.wikipedia.org/wiki/AirPort_Extreme#Comparison_Chart
Looks like a two year primary design cycle. So likely 2013 (which makes sense. 'ac' should have somewhat matured at that point. )
Express
2004
2008
2012
http://en.wikipedia.org/wiki/AirPort_Express
Four year design cycle.
Time Capsule has been a bit of the odd-ball compared to the other two
2008
2009
2010
2011
I suspect though that Apple has managed to synchronized with the Extreme so that they will come in pairs ( new Extreme comes with new Time Capsule) since the same core platform in different cases. Or that the intermediate Time Capsule deltas only track changes in the HDD components; not the networking. Given the disruption in HDD supplies I doubt there is incentive to increment sizes this year. So 2013 is likely for it too.
Until Apple starts shipping Macs and iOS devices with "ac" in any quantity there isn't a big demand for the networking devices to follow. I'm sure there is one in the "lab" being worked on. Just doesn't seem likely it will appear in 2012.
Similarly, I'm not so sure that 'ac' is going to be highly effective in "home" deployments. There were several questions asked in the thread on whether 'ac' would decrease interference. It doesn't. In fact it cobbles even more of the spectrum than 'n' does. That's nice if there is plenty of "excess" spectrum to share (neighbors are many yards away) . It is not so nice if in a dense "hot spot" location ( the list of available networks on your client devices is relatively long)
How to place 4-7 of these in relatively close proximity of each other is actually a harder problem if going to interact with a range of 'g' , 'n', and 'ac' devices. Or even just 'n' and 'ac' devices. If the airports are going to try to "automagically" do that for you it is going take Apple time and testing to work through the permutations.