Never.
It is a Steam-only game after all, and I'm surprised that a lot of people seemed to miss this.
Modern Warfare 2 isnt Steam, but you're right, according to Infinity Ward, that it will be released for the PC only using peer-to-peer like the consoles. No dedicated servers anymore. This has caused quite the ruckus in the PC gaming community, with almost 200,000 signatures on an anti-MW2 petition. Reportedly, Amazon alone had over $200,000 in preorder cancellations after IW announced the change in concept.