They're not making Episode 3 for a while aparently, they want to get community feedback first on where they should take the series.
Which means 18months+
Quote: "For one thing, EP 1/2/Portal do not *require* HL 2. They're sequels to HL 2, not expansion packs."
Well that's a matter of perspective really, as they install the Half-Life 2 game just to play.. if you get it on DVD and play offline, then you'll already have Half-Life 2; if you get it from online then it already installs it.
Although I'm constantly confused by the engine updates, wish they'd apply them immediately to the previous games. It's like the HDR, god knows why they couldn't set it up so it would be backward compatible. Playing through Half-Life 2 with bloom is cool, but HDR looks better... I wouldn't mind using the new soft-shadows and such.