Having high requirements is one thing, not supporting XP is just asking for trouble. I could see that happening only if XIV for some reason was completely reliant on DX10.
Only once a very solid Windows 7 base has been established will developers begin to break off support for XP, as most people are wisely skipping Vista entirely.
Actually in point of fact, Microsoft will be halting support for XP sometime close to the release of FFXIV (this is assuming we're looking at a Q3 or Q4 release of FFXIV). Windows 7 will be officially available on shelves October 22
Admittedly, we'll probably see games released on Windows XP for a good bit after that, but it's not totally
out of the question for FFXIV to not run on Windows XP. From personally experience, Windows 7 has a tremendous amount of potential; in all likelihood, we will see it overtake Vista in installation base within months, possibly overtaking XP by the end of the year.
SE has said that they would like to take advantage of the most advanced technology available at the time of development. I see no reason why they wouldn't make extensive use of DX10.1 and better functions.
I'm not really saying that FFXIV will not support Windows XP, just that it's too early to discount the possibility of it being built for Windows 7 instead.