I agree with most here that validated HTML pages are a plus, but not a requirement or a ranking booster.
A fair assumption about validated pages would be that they would load quicker & with less problems, both for users (which Google likes) and for Google's spiders. The less potholes in the road, the more smooth the drive is.
While being "validated" is certainly noteworthy, I think that having a cleaner & faster loading site is actually why you might see a trend of higher performance (rankings).
I actually blogged about this exact topic this morning.
(note to mods: this is not pimping my own site, just providing additional support for my comment on this topic)
The example of Wikipedia is completely off target, since a lot of their rankings are a result of on-site content and the internal linking between their pages.