I hate these "mini-factlets" that people keep repeating. Just because something used to be true doesn't mean it still is true, no matter how convenient the factlet happens to be.
Windows 7 is the biggest user-desktop and has been for several months now. http://www.ubergizmo.com/2011/10/windows-7-surpasses-windows-xps-market-share/
Ps, if you want a good reason to upgrade, I still think ASLR (http://en.wikipedia.org/wiki/Address_space_layout_randomization) is my favourite (and least well known one). It massively improves the security of your entire system, from the kernel, to the browser, to every app you choose to run on it - and is the reason why almost every single publically released exploit is either a logic-flaw or is targeted at Windows XP.
But that's total market share. I believe that a large majority of consumers have upgraded or purchased new PCs, but a lot of businesses are sticking with XP because there are no compelling reasons to upgrade. Security is better, but our users are locked down and get a fresh VM every login (thin clients connecting to VMware VDI). Thin clients also remove the reliance on driver support.
Word, Excel, and homegrown VB6 apps don't benefit at all from a move to 64-bit.
As much as I'd love to be running the latest and greatest, there really is no business case for the upgrade. What I would like is to be able to develop in VS11, but I guess that's not going to happen.