I'm pretty sure it'll be a matter of Trey & Matt not wanting to do it anymore rather than Comedy Central canceling it. CC will probably keep it going as long as T&M want to do it.
If you listen to the mini-commentaries they do on the DVDs, it always seems like they're talking about dreading the deadlines and struggling to come up with ideas and they act like they hate it when they're doing it but always want to come back to it. In particular, if you listen to the commentaries on the episodes from seasons 3 and 8 (the years they made the South Park movie and Team America, respectively), they sound like they were especially stressed out during those seasons. I have a feeling it's probably the same case for the past year and this upcoming half-season because of The Book of Mormon.
That fact, combined with the fact that it's the last year of the contract, and the success that Mormon seems to be having, makes me wonder if they'll finally say "enough is enough" and decide to move onto other types of projects like Mormon (which sounds like it could be a kind of "graduation" for them, if that makes sense). If that is the case, I hope they don't decide suddenly towards the end of the season, but earlier on so they have enough time to give it a proper sendoff.
Or they could sign another 3 year contract and keep it going indefinitely. Who knows.