[Christian]

In my understanding, if you sync OF with another WebDAV server (like omnisync) the. OF copies it's database to that server. Next time you sync to the same server it checks for changes and updates the database on the server. If you use more than one device with OF on it and sync it with this server than it also checks for the newest changes and applies those to the other devices.

If you want to just change from MobileMe to omnisync, no issue. If you want to swap between both on a regular basis and just use one device (so kind of a backup), still no issue. If you use more than one device then I recommend deciding for one service since it can be pretty frustrating if you sync your iPhone with MobileMe and then the iPad with omnisync just to find out that, of course, the changes you synced to MobileMe from your iPhone are not on the omnisync database and hence do not make it to your iPad.