Steam is currently updating my copy of CS:S beta with a 3+GB download...
Good way to come back from vacation.
This is why...Counter-Strike: Source Beta Reopened
Product Update - Valve 15:20
Starting today the Counter-Strike: Source Beta will be available to all PC Counter-Strike: Source players. As we continue to update CSS and address issues, we want to make sure to not destabilize the core game. By releasing updates to CSS Beta first, we will be able to solicit community feedback before those changes are applied to CSS.