Looks like Turtle Rock Studios is back in business and working on new projects.
The Left 4 Dead co-developers, previously purchased and split apart by Valve in 2008, have regrouped and started hiring staff with Michael Booth heading things.
Turtle Rock originally started working with Valve in 2005 after the firm was impressed with TR's handling of CounterStrike ports and expansions.
When purchased fully in 2008, it was renamed Valve South until Left 4 Dead was released and Valve asked staff move to headquarters in Washington state (several did), which saw the closure of Valve South.
No word yet on the projects being created at the studios, but it still maintains an amicable relationship with Valve.