I don't see why you couldn't simply pick up HL2:episode 1 without playing Half Life 2, yes you'd lose out on story, but it is it's own self contained game.
I'd imagine episode 2 would be similar. The real reason to buy episode 2 now is because it also comes with Portal and Team Fortress 2 - both should be amazing stand-alone games in their own right. Odds are they're going to try and sell episode 2 as a combination of all 3, then people will play episode 2 and go back to buy HL2 and HL2:Ep 1 if they haven't already got it.
Of course, if yuo haven't already got HL2 yet, you happen to be missing out on one of the better games in the past few years.
SixthAngel: Although it seems you already did the research, portal is done by the same guys who did narbucular drop. Part of the reason it's so impressive.