PC gamers won't like this.
One of the greatest things about PC gaming is that people can set up dedicated servers for multiplayer matches. None of that silly peer to peer stuff that might give the host an advantage, not to mention that dedicated servers typically have connections that are capable of supporting a greater number of players with smooth and stable results.
Unfortunately, it seems that Modern Warfare 2 won't be having any of that. Joystiq heard on the from Infinity Ward's Ryan Bowling on the Bash and Slash podcast that matchmaking within Modern Warfare 2 would be reliant on proprietary server technology (called IWNet), and wouldn't allow users to host their own dedicated servers.
While those playing COD4 on the Xbox 360 and PS3 have plenty of fun without the need for dedicated servers, there's still a lot to be said for having dedicated clan or local servers where everybody knows your name.
On the bright side, the PC version isn't delayed.