The impending end of support for XP has done quite a bit for the small business I work for already. For most residential customers that still have a working XP era system, Linux Mint has been an easy transition. The difficulties have been few (support for older Brother and Lexmark printers, out of box suspend not working with some hardware configurations, multi-monitor support although improved still needs more work, lack of easy options for iTunes pruchasing/updates, and Netflix subscription). Things have looked better in each of the standard releases, but we're sticking with the Mint 13 (Maya) long-term service release for ease of support. I have introduced customers to some of the other options (Redhat, Ubuntu, and in a few cases ArtistX) but most customers have opted for Mint.
With the nature of driver support, older, pre-Vista era, systems have required fewer tweaks (Broadcom firmware for wireless for one example, although searching in synaptic for b43 makes that easy enough to fix). Even learning when to use nomodeset or the various ACPI options for finicky hardware has been easy compared hunting for drivers when they either aren't present (I'm looking at you Lenovo and Toshiba) or haven't been updated since the machine stopped being under extended warranty (*cough* Dell *cough*), or the driver is listed under the wrong operating system more than a few times (HP/Compaq and Dell again).
Reinstalling XP on 10 to 20 machines a month, I've gotten a glimpse of why the companies wouldn't want to put the time and effort propping it up any more. Much like Vista, XP still works fine until something requires you to reinstall (such as hard disk failure) or to move on (oh, you wanted to transfer your existing licensed software onto that brand new system? *snicker*). Transitioning from one version of a software program voluntarily makes for happier customers than if they find out there's a looming need to switch.
I think XP will probably be fondly remembered as one of the better operating systems overall, but enough technology has continued to happen since it was released that it's time for some other contenders to enter the ring.
"The fundamental principle of science, the definition almost, is this: the sole test of the validity of any idea is experiment." -- Richard P. Feynman