The promised Team Fortress 2 beta went live overnight on Steam, presumably before
midnight Pacific time to fulfill the promised September 17 release. The beta is
open to those who have pre-purchased the Orange Box Half-Life 2 bundle.
Demos are not full products. Betas often are. This is.
Demos often aren't the best idea for games that are mostly multiplayer, anyway. Look at the gamespy stats - some games (like Halo) have their demo still tearing up the charts long after the game disappears.