I suspect that the survey is skewed in other directions when it comes to large corporations, though. I would expect to see much more Perforce if the big not-primarily-computer-companies were included, because for many years Perforce was pretty much the only game in town for large, multi-million-line codebases. I still can't imagine Lockheed running their sourcebase off Subversion or VSS; even when I interned at a mid-sized (~100 person) government contractor, we used Perforce.

I think the survey is representative of the geographically-distributed, Joel-on-Software reading, micro-ISV crowd, and not much else. Micro-ISVs often develop on Windows (because that's what their founders are used to) and use Subversion (because it's fairly easy to setup and generally adequate for 1-10 devs). I suspect you'd get very different numbers at YCombinator or Google or Lockheed, though.