Since no one has replied to this I figured I'd go ahead and try it out myself. The PPC driver installs fine on my X50v upgraded to WM5, and they keyboard works perfectly. However, I do get an annoying error about power management every time I restart the driver, which equates to every time I soft reset my Axim! At least it doesn't seem*to affect keyboard operation at all, so I'm happy.
Obviously palmOne hasn't officially said anything about WM5 support, and their helpdesk guys are clueless, so try this at your own risk.
