I don't know... MS bashing aside - I'm tempted to side with Stuart on this one.
Microsoft NEVER released USB2 drivers / device support for Windows 98... EVER... the manufacturers had to do their own drivers... this is why under windows 98 the USB2 device names vary so much from manufacturer to manufacturer (e.g. high speed USB etc etc) - there was no standard device name / device support, since the manufacturers all had to develop their own extensions....
Microsoft only produced drivers/support/updates related to the USB2 release on their most recent offerings in each category of use at the time (Windows XP for desktops, Windows 2000 for servers)
Do you honestly have reason to expect different when the time rolls around for USB3 support to be implemented that MS will do differently?
So depending on WHEN USB3 comes out, the probability is that support will only be implemented by MS for Vista (on the desktop) and then... either Server 2003 or Vista server (to cover the server market)
Oh and don't you be thinking you'll just install that windows server 2003 update on your XP machine (if USB3 comes out soon enough) if history holds it will be part of a service pack, so tied to server 2003.
That's not to say you won't have USB3 support on XP - it will just be provided by manufacturers and so will be non-standard and therefore somehow not as integrated as the MS provision.