According to Infinity Ward community manager Robert Bowling, they will never charge for online play.

This rumor has absolutely nothing to do with Infinity Ward or our games, including Modern Warfare 2. We would never introduce such a system for Modern Warfare 2 so don’t worry.

Multiplayer will always be equal and free for everyone as always.

Certainly, someone above him could veto his policy, and even if someone higher had said it, they could always change their minds later. Nonetheless, this should provide some comfort to those worried that there would be fees to play online.