I'm going to be a bit of a contrarian here:
First, regarding being able to tell the difference between a high MP back and a lower MP cam, even in small web prints. Sorry, but I just don't buy it. It doesn't jive with my own experience, and as Lisa_r points out, we've also heard comments from Michael that experts couldn't tell the difference between Canon G10 images and P45+ images at a certain print size. If both files are properly processed, you won't be able to tell the difference in smaller prints, assuming you use good glass and good technique with both. I think what we're really seeing here is psychology. People see what they expect to see or want to see. And when you pay $40K+ for something, you expect and want to see a lot.
Second, I find it tremendously disappointing that at this stage of the game, Phase is still just jacking up MP without really fixing some of the underlying problems with their design. OK, maybe they've improved noise somewhat (tho I'm still withholding judgement on that, since I think that's also a candidate for psychology at work). But they still have that appalling LCD screen. They don't have any real live view (yes, they have it with a laptop attached, but that still sucks). For something with this kind of resolution, used out in the field, I'd want to focus via live view at maximum magnification, rather than relying on AF. Having to use a laptop for that on landscape shoots just isn't practical. The back still sucks power from the batteries. Only 100 to 200 shots to a 2500 millliamp hour battery is just terrible. For $40K+, these shortcomings shouldn't be present.
I understand Michael's enthusiasm, but such unbridaled enthusiasm is common among people who've just bought a hugely expensive product and expect the best from it. This is human nature. Given that this is for all practical purposes just a MP bump, I'm underwhelmed.
P.S. Of course, this is just my opinion. And you know what they say about opinions :-)