It looks like they're going to be merging a bit of the Portal story with Episode 3. The general consensus I can tell from the gaming press is that Episode 3 is going to be the last of the HL2 episodes (this may well have actually been stated explicitly) and that there will be a Half Life 3. After all, Valve wouldn't want to give up their baby, now would they?
It's a great game with some very interesting story behind it, and I always have a lot of fun charging through the new eps when they come out. It's a shame there is such a big gap between each release though.