From a practical standpoint, I blame the L4D series for the delay. While it's entirely possible there are completely separate teams working on L4D and Half Life, it's still a timing issue as Valve does not want to flood their releases, instead spacing them apart to prevent its own IPs from competing with one another.

It did strike me as being off that they did back to back L4D games. L4D2 could have been a big expansion pack given the quick turnaround time, so maybe that explains a lot. 

From an R&D standpoint, as it's already been suggested, Valve is probably going to debut a rewritten version of the Source engine with Episode 3.

I also like the idea that there will be convergeance between the plotlines of Portal/Aperture and Half Life/Black Mesa as strongly hinted at in both Episode 2 and Portal.

So chronologically, players will want to play through Portal 2 first anyway.

Gosh....I don't like Portal very much and really don't want them to be some hybrid mash-up with Half Life 3.

how can u not like portal o.O it was awesome ness XD

It's just not my kind of game. I know it's a good game and it's awesome. It's just that I don't like it.

fair enough, suppose it was a kind of weird game :P