Saw on Bluesnews that Gamespy has a short question and answer session with Doug Lombardi of Valve.

Q. Are there any plans for a standalone Half-Life 2 multiplayer part, deathmatch, co-op, somewhere down the line?

Doug Lombardi: Nope.

Get the picture?

Anyway, they do briefly cover the fabled Teamfortress 2 and a few other bits and pieces, so it is worth a look.