Source: HL2Files.com

HL2Files.com reports that Swedish Gaming magazine broke it's NDA and reported the following:

Speaking during a recent press briefing at Valve HQ, Newell is said to have revealed that, at this stage, the company envisage Half-Life 2 female NPC Alyx as playable in Half-Life 3, and promised more alien-based revelations.

So Half-Life 3 and or a expansion for Half-Life 2 are possibly already being set up. Do note this is in no way official. You can read the full story here.