I'm gonna go with 26.5 million and let me tell you why.
As noted in the OP, MS sold 8 million in the first 60 days, 2 million in the last 60. That rate will continue until Holidays unless something major happen. I already hear you saying "well that rate won't get it to 26.5 million". Well, I said unless something major happen and I expect something major to happen circa E3. Sony will drop the price of the PS3, that's a fact, everyone is expecting a U$50 drop on the system, but I'm expecting U$100, if they were gonna drop it by U$50 they would've done it last Holiday. No, they'll drop U$100 and put the pressure on MS (and to a lesser extent, Nintendo). And even though MS is making a killing on the 360 in NA, they'll need to do something.
Now, the major, major thing I think will happen is, I think MS will do away with standalone "Elites" and "Cores". That is, I believe that, starting this Fall, all 360s will come with Kinect. Core Kinect bundle will drop to U$200 and Elites to U$300. Kinect standalone will drop to U$129. Add that to hardcore games that are coming (Gears Kinect, Armored Core, possibly more) and I personally think this would be a master move by MS and I think they'll go for it.
BTW, I'm lowballing my number on purpose, don't be surprised if it exceeds that number.
