I'm told (though not from an authoritative source) that booting XP does a large number of small disk writes, which will wear out flash memory within a fairly small number of boot cycles. I don't know if "small" is on the order of 5, 50, 500, or 5000, but the guy I talked to said not to bother.
XP-Embedded has a feature called "enhanced write filter" that was added specifically to address this problem. Much as I like the idea of running without disks, I'm going to wait for XPe first.