Kikizo have flared up an interview (3 pages long) with Doug Lombardi of VALVe Software regarding both Left 4 Dead and some small tid bits on Episode 3:

Kikizo: When are we going to start to hear about Episode Three? Because the gaps seem to be quite long based on the first couple of episodes.

Lombardi: Yeah, the next time you play as Gordon will be longer than the distance between HL2 to Ep1, and Ep1 to Ep2.

Kikizo: Won't you announce or show anything on Episode 3 this year?

Lombardi: We may at the very end of the year.