Call of Duty Black Ops was released in November of 2010, and has gone to have become the best selling game of all time. [source] Some say Black Ops sold over 25 million units thus far. Treyarch released 3 map packs thus far, and nothing seems to be stopping them.
According to Loudmouthed Gamers, OXM let it slip that Treyarch is currently working on their next Call of Duty title. They quoted OXM saying “What will Treyarch’s 2012 Call of Duty game be? Bet on Black Ops 2 says The Ball…”
While it’s quite too early, OXM is a very credible source, so I would have to believe the next game is Black Ops 2.