Not intentionally, but they are obviously biased towards the xbox. Which is understandable because it is the most popular, its the easiest to create games for, and some will disagree with me, but like I said, you can only make a game as good as the weakest system, so you design it for xbox. 90% of multi-platform games are created this way, but usually there aren't as many problems.
But right from the start IW made statements that showed them playing favorites. Exclusive content for the xbox, a modern warfare edition xbox, commercials saying "modern warfare 2 runs the best on the xbox" and I'm sure I'm forgetting things.
I haven't personally had any major problems, so far I'm happy with it. Only problem I had was the invite system not working and not being able to get online for a few hours a day after launch. I'm just going off of other people's problems.