I think that people need to look at the applications they use before they do any assessment about speed, because the applications are what you will be dealing with, you're not going to be staring at the OS all day.
If you read the recent PCMag review of the Mac Pro Nehalem 8-core, for example, you'll see that they found that many applications were much faster in BootCamp than in OS X itself. Photoshop CS4, for example. The main reason being that the Windows version can address 8 GB of RAM while the OS X version only gets up to 3.5 GB. This is not Apple's fault, of course, it's all on Adobe, but the end user doesn't really care about whose fault it is, all he knows is that he's getting subpar performance in OS X from the application he works with all day.
I use Cubase and Reason a lot, and I could easily switch from Windows to OS X any day since both apps are cross-platform. But Cubase is much more unstable on Mac than on PC. Again, not Apple's fault, but that's not going to ease my problems when I'm on a tight deadline and I have a responsibility to go with the most stable platform for Cubase, rather than call the client the next day with some pathetic excuse about crashes and ask for a revised deadline. Reason performs better on PC, too, if by a narrow margin. Again, not Apple's fault. But I need all the performance I can get. If it's between thinning out a musical arrangement because it chokes the CPU in OS X, or keeping it exactly the way I want it by going with Windows, from both a creative and a practical standpoint Windows would be the way to go.