As reported here back in May, ASUS will offer Windows Mobile 6 upgrades for a number of its PDAs, namely the A639, A626, A686, A696, P535 and P735. Some of these upgrades are now available on the ASUS support web site. You can now download the upgrades for the A626, A686, A696, and P535 (I only saw a Thai version) versions.
I did not find upgrades for the A639 and P735 so if you own one of these, keep an eye on the ASUS support site.