From Steam Support's page on VAC (down right now because of the shit they've been hit with, from a Google cache):
VAC Enabled Games
Valve games
The Valve Anti-Cheat system is enabled for servers displaying the Secured shield logo. Using custom skins, sounds or maps and playing multi-player mods which do not modify core .EXE and .DLL files will not result in a VAC ban
Cheating in one of the following Source games will result in a VAC ban for all games in the list and your items from Team Fortress 2 will be removed except any items that have been purchased:
Counter-Strike: Source
Half-Life 2: Deathmatch
Day of Defeat: Source
Team Fortress 2
Cheating in one of the following Gold Source games will result in a VAC ban for all games in the list:
Counter-Strike
Condition Zero
Ricochet
Day of Defeat
Team Fortress Classic
Half-Life: Deathmatch
Deathmatch Classic
Note:
Excluding the games listed above, VAC bans will only apply to the game the cheat was detected in. For example, a VAC ban in Modern Warfare 2 will not affect Modern Warfare 3.