Nickster from Half-Life 2 Source has interviewed Gabe Nwell, Chris Bokitch and Dhabih Eng form Valve and he has managed to get some new info from them.

Nickster: Will HL2's multiplayer support over 32 players?
Gabe Yes.

Nickster Will we have to wait another 5 years for HL3?
Gabe I have no idea. Its a question we get to think about once we'er done with HL2.

You can read the Interview Over at Half-Life 2 Source