I think that Vista was exactly what they planned it to be, and XP was the 'oops'. Microsoft made the mistake of giving XP those "forgiving" hardware requirements (I've seen it run on a P2 233MHz with 128 MB of RAM and a 4 MB Video Card) instead of having it take the specs a step further. That was why Vista was such a huge "upset", was that it was too much of a leap from XP, there was no middle step because XP didn't take that placeholder. If you look at 2000 vs Vista, and XP vs Vista, it's pretty much the same thing. They have almost the same differences. Really, XP should have been brought out about two years later, and should have had a bit more 'weight' as far as hardware, and we wouldn't be whining about Vista now.
There's no doubt in my mind that 7 will be evolutionary, but the question is, would it really be that evolutionary if XP had been a real step towards Vista? I think not, because in that case we'd still be on the same upward track.