Peng, please email
omnifocus-iphone@omnigroup.com with the details of just what you were doing when the webdav data was wiped out. The upgrade process doesn't touch the webdav server, of course, so any data loss on the webdav server would have to be related to subsequent OmniFocus syncing (or something else impacting your webdav data). To the best of my knowledge, we have not had anyone else report that their webdav data was simply wiped out unless they chose the "use local data" option when prompted on the next sync, so if there is a bug, we would definitely want to try to identify it so we could address it.