Gabe said there is a really good reason why we haven't heard anything about Episode 3 yet. As there is no one in the industry that I could trust more, I think this means that it is either A. Going to be epic, much bigger than the previous two episodes and be all around totally awesome...or B...see above but actually is Half-Life 3 in some way...either by an updated source engine or something.
Personally I don't think it'll be 'Half-Life 3', as that will hopefully be made as a new chapter entirely in Gordon's story. We're still looking for a conclusion to Half-Life 2, and that to me does not say a new game...but quite possibly a third episode that is as large as Half-Life 2 itself.
What have the previous complaints been about the episodes? Waiting too long for too short of a game, and no new innovative weapons. My bet is we get both of those, an epic storyline and conclusion that answers a question or two, while raising many more. And maybe some babies. You heard the doc, GET BUSY.
And I think we'll get it this year. They like to surprise us.