Between the Steam Box, porting everything in their catalog to Linux, Dota 2, and scrambling like madmen to make the ten thousanth TF2 hat, I assume Valve has a lot on their plate right now.
Roll out a new revamped set of TF2 Unusuals and, boom!, cash enough to hire another entire product team.
It's presumed (and pretty much a given) that it's being worked on. However there has never been a full confirmation from Valve/Gabe Newell on anything specific.
I was really looking forward for this, I am old now but not old enough for HL3.