There's a lot going round about this, a lot of people are obviously for it. Others are dead against it.
RockPaperShotgun especially have an interesting comments thread (that I also had to stick my oar into) about the whole thing. People do have some compelling arguments on both sides.
Most of the "gaming news world" appear to think this is all well and good. My personal feeling (as a consumer) is that it's going to bite them in the arse. My prediction for the sales of L4D2 are as follows:
PC version: fewer than expected
x360 version: sky high
And I would think that's what Valve expect too.
Valve managed to get an update onto the Xbox version for L4D1 for free. As Doug Lombardi (Valve Marketing) has said "this wasn't easy". And it think this is the whole crux of the matter. To release DLC for a price is against the norm for Valve. But creating a "sequel"? Well, that fits in with their episodic model.
So, for xbox users who are used to paying for these kinds of updates (I'm not saying it's right, I'm just saying it's part of the xbox way) this isn't going to be too much trauma for them. However for PC users this is going to make a huge difference to the sales figures.
I can understand where Valve are coming from, but it's astounding that no PC user they initially asked about this idea (asuming they did) said "Um, wasn't all this supposed to be coming in L4D1?"
Overall I'm not too worried about all of this. Valve have always had good years and bad years in terms of public opinion. Remember when HL2 got delayed? Those were bad days too but they turned that around eventually.
Luckily there's something to look forward to from E3 at least. Gearbox's Borderlands sounds like it's just as good as I would have hoped from reports I've read. I only wish I'd been there to do some reporting.