Like I did with Black Ops, I'll wait for reviews.
Once the Black Ops reviews came in overwhelmingly positive in regards to the multi-player, I traded in MW2 and bought that. If it's the same case here, I'll trade in Black Ops.
With the Call of Duty games, I get really into them for several months and then my interest drops off completely. Once I beat the single-player and get through 1 or 2 prestige levels online, I seem to stop playing them as much.
I am leaning towards getting Battlefield 3. If that drops before MW3 and I actually like it? I might just miiiiight skip MW3.
I've been considering that too. Will wait for reviews though and see how the replayability/longevitiy of the online is for BF3.