Aliens vs. Predator released today, and PC players are stuck with peer-to-peer matchmaking for a little longer while Rebellion gets a dedicated server ready "in the next few days," according to the studio.
In a Steam forum post today, Rebellion had this to say:
Rebellion are busy preparing the dedicated server for the Aliens vs. Predator community. We hope to have a beta release with reduced functionality out in the next few days and further updates to improve functionality and fix bugs over the next few weeks. The files will be available to download from the "Tools" tab in the Steam client and will be a free download.
If that's not good enough, well, that's what the singleplayer campaign's for, I guess.