Actually, Valve people say that the development of the first and the second episode began at the same time, so we don't have too much time to wait before the second episode will be released.
Valve is not going to release big and expensive expansions but small and cheap ones that will be available through Steam.
We can only guess that Episode 1 + 2 are the two parts that the original expansion Aftermath was supposed to be consisted of, but will never be released.
Some more info:
http://www.gamespot.com/pc/action/halfl ... id=6144642
