I found this in the Linux-Wacom-Project ChangeLog:
2006-03-06 Ping Cheng <email address hidden>
* Fixed a Volito bug (Kirill Frolov)
* Fixed a xsetwacom.c typo
* Supporting tablet attach/detach while X driver running (Christian Lupien)
* Remove SYSCALL from xf86 calls (Christian Lupien)
* Swapped USB tablet wheel directions (Christian Lupien)
Notice the last line... could this be the reason?