I think the word of mouth reviews will push this expansion into 3.5 million units in the first month. There was a lot of skepticism going into this expansion but i think it has been fairly well received.
Blizz is going to have a very hard time of attracting new players at this point and old players have moved on with their lives and/or interests. At 10 million players it is still absolutely massive, even if it loses 50% of its player base it will still be absolutely massive.
If they hit 3+ million copies in the first month I think they'll be happy. I'm predicting 3.4-3.6 in by 10/25.