Not every processor has the features to use XP Mode; many processors sold today can't run it. If you have an existing system, you can download the SecureAble utility from Steve Gibson. It will detect whether the processor supports hardware virtualization, the feature needed by XP Mode. If you're buying a new processor from a retailer like NewEgg, their product information indicates "Virtualization Technology Support" in the listing. It may not be quite as easy to tell the score for OEM computers; you'll need to look up the exact processor model and check the AMD or Intel site if the OEM doesn't say anything about virtualization support.
I wonder how many (and how quickly) businesses will upgrade to Windows 7 if Windows XP compatibility is a high priority. Since many current processors don't have the virtualization support required for XP Mode, simply upgrading an existing PC to Windows 7 won't solve the problem for many (if not most) businesses. Updating the operating system on an existing older PC has its own set of challenges anyway, especially since Windows 7 won't support an upgrade from Windows XP. If a business wants Windows 7 but also needs XP Mode, the most practical course is to buy a new PC with a new processor that supports hardware virtualization.
Finally, there is the problem of Windows XP dependency. Microsoft's current timeline has XP reaching the end of its life in 2014; that's the point at which Microsoft won't offer security patches for it anymore. XP Mode in Windows 7 will continue that dependency for several more years; Microsoft will be sanctioning, perhaps even encouraging, its use. That will most likely lead to a last-minute reprieve for XP support come 2014, even if Windows 7 is wildly popular.