I can only guess they're actually working on something new for Half Life. Episode 3? Half Life 3?
Really, I'm not particularly antsy. I can wait. Though, that's really all I have to look forward to from Valve at this point. TF2 doesn't do much for me... whatever I loved about TFC is no longer present. I found L4D2 pretty un-exciting, bordering on tedious. I've always disliked Counter Strike, so this new one doesn't appeal to me at all. Portal 2 was brilliant, though I don't see a third one coming along any time soon.
So, whatever. Valve, do your thing with Half Life. I'll buy it when it's ready. Until then, I'll be playing Deus Ex, Mass Effect 3, Diablo 3, and maybe Skyrim (when the latter three are released).