Valve plans to release the Sniper update for Team Fortress 2 in "the coming days".
Inside will be three new unlockable weapons, extra Achievements and some maps. Valve will drip-feed specific details on the Team Fortress 2 blog over the next few days.
We do know one of the new weapons will be The Huntsman bow and arrow, which pins prey to walls. Let fly a bolt at someone's head to score an instant critical hit, but beware of the one-second full-power charge and the limited quiver of 18 arrows.
This, like previous class updates, is only for PC. The whereabouts of the "massive" Xbox 360 patch, which will contain all the PC updates so far, is still unknown. Valve has confirmed it to be in development, though.