Hi
In the video editing and audio production forums I hang out in I've seen quite a few people purchase '09 Mac Pros, a steady ongoing trickle. That's probably more than I ever saw in the tower G5 or early Mac Pro era.
In the real world of media production no one even considers a hackintosh. The people who (re)turn to Windows are recent switchers to Mac who learnt Windows at a young age and can't give it up (their loss

)
What I see everywhere (and I am one of them, a small owner-operated production setup) is G5 tower users still going strong on their old kit....
Yeah, that's a point too. Machines these days "can do" for longer periods. It used to be that a new wave of machines (usually every 2 years or so) meant almost 2X speed and performance increase and users who didn't upgrade were faced with production disadvantages or even software upgrades that couldn't move. For the past 6 or 8 years upgrades have been in much smaller increments - maybe a 0.3X from machines of THREE years previous and almost all of the software that worked OK when it was new can still work fine on the older machines after upgrading those apps. There are some production disadvantages but they're not nearly as pronounced! How that is affecting the difference between 2008 and 2009 sales numbers I have no idea and and couldn't guess the specifics.
In the video, audio, and CG computing segments of those industries of which I'm a very big part, I however, am not seeing what you describe. Yeah I see the constant trickle you mention, but most people in those areas of production don't choose a platform because it was something they had as a kid. They look at the software environment they need to submerge themselves in (in order to be competitive and make money), consider the price tag, and make the decision. Unfortunately this rationale leads 80% to 90% of all buyers in those industries to choose Windows - and there's no sane argument to the contrary. They need Motion Builder, Digital Fusion, mature Houdini, XSI, etc., and a thousand free support apps all ready to go. With Apple or Linux they have a vastly reduced selection and only one or two (sometimes) free support apps. There's more "support" apps available for free on Linux than there are for mac actually.
In 2006, 2007 and 2008 Apple's factory assembled hardware kits were not over-priced! For all of those offerings I had a hard time beating the prices Apple was asking. The only way I could do so was by cutting corners and buying different parts. The alternative parts contributed every so slightly to instability but nothing deal-breaking. So if one wanted a fine grade system at a very fair price Apple's Mac Pro's
were indeed a consideration. And it could run Windows at nearly full speed if needed too.
In 2009 this is no longer the case. Apple has decided to add between $1,300 to $2,200 to the base price of IDENTICAL off-the-shelf parts and to their own previous line-ups. They have additionally retained 2006 specs or actually downgraded components over what is typically offered today in 2009. For what Apple is asking, one can purchase faster RAM, faster motherboards, more HDD and USB connections, more RAM slots, more PCI card slots, and still have money left over to buy 8 or 12 extra gigs of memory
and a second monitor over what the Apple comes with. None of these things could be said about the Mac Pro's offered previous to Apple's 2009 line-up.
It would only be logical to assume that the reasoning explained above are the primary factors and drivers that would cause people to (re)turn to Windows or Linux. It's simply no longer a good deal. It's not even a fair deal. Thinking that
industry professionals are selecting an OS based on reminisce or familiarity is slightly absurd. Especially when you consider that for most professionals, it only takes a few hours to learn the differences between any of the platforms Windoze, Mac, or Linux. They are after all, primarily concerned with application operation and technique.
Again how this all affects the "desktop" sales records for Apple or the distribution between models which I guess includes the Mini's and iMacs, is entirely unclear to me. I would assume that the computer buyers described here would not consider an iMac or a Mini as an alternative. I would assume the exact same things that have always been the primary issues for professional users: The price

erformance ratio and the required Software environment. In 2009, Apple loses big-time in both departments.