When I'm in the office (75-80% of my year), i'm a heavy user of Windows. I cycle between five core apps (Word, Excel, Visio, PowerPoint, Outlook+Lookout, VMware/VirtualBox) working on RFPs, Technical Architectures, Transition Documents, Reference Designs, etc...
My "travel system" has changed three times in the last 9 years (Macbook Pro, Macbook Pro, MacBook Air) - but my productivity desktop has remained the same - a Dell Precision 650 running windows XP. I'm _already_ looking forward to my fourth laptop (picking up a 2012 thunderbolt MacBookAir - local backups over a thunderbolt connection to a high-speed NAS will make local backups both more likely to happen as well as more painless) On the flip side- my circa Q1 2004 productivity desktop _still_ does pretty much everything I need of it - I don't have any real incentive to request a new machine, or upgrade off of Windows XP.
I'm picking up a new iPad on Friday, but I don't really see how Windows 8/Metro is going to be a useful replacement for my fairly optimized Windows XP experience. Eventually the Precision 650 is going to break - and I'll probably upgrade to Windows 7 + whatever dell desktop will last me another 10 years, but I agree 100% with the parent - Mobile/Tablets/Laptops still have 2.5-3.5 year lifespan, desktops have moved into the 4-6 year rotation in the enterprise (And, in my case, even longer)
As the world becomes more mobile, and desktops continue to extend their life, we'll see even more transition of leadership (and profit) to those vendors who focus on the "Mobile Experience" - that's what's driving Microsoft to Metro - not because they believe it will enhance our desktop experience (it really, really won't) - but because it's where the market is moving.