Just incase anyone else is wondering why this has appeared in their game’s list on Steam, Stalker kindly pointed me the the right direction last night.
http://www.pcgamer.com/2010/12/03/team-fortress-2-launches-public-beta
Also, as always the TF2 Blog – http://teamfortress.com/
What sorts of things will we use the beta for, you ask?
Testing and tuning various class, item, and weapon changes
Testing map changes and variants
Running higher level, game-wide experiments (like doubling player health)
Testing new technologies without the risk of breaking the game
The beta is available to anyone that owns a copy of TF2. To install, launch Steam, open the Library and install “Team Fortress 2 Beta” from your games list.