[Andrew]

Yes, you can do this, and it's a great way to have a backup of your OmniFocus data in case anything happens to your phone (or if the phone accidentally wipes yours OmniFocus data when you install an OmniFocus update, as has happened to some users).

I'm assuming that you already have a MobileMe account; if not, you'll need to go to me.com and sign up for one. (They have a free trial period, but after that there is a yearly fee, which some people find too expensive just for OmniFocus backups. In that case you can try any hosting service that correctly implements the WebDAV service; unfortunately other providers are not necessarily cheaper although some are, a number have flawed implementations, others can be somewhat complex to set up, many do not provide HTTPS for WebDAV, meaning that your OmniFocus data is not secured while it's being transferred, and some do not even provide Digest authentication, meaning that not even your account login information is protected when OmniFocus is logging in. Sorry for such a scary disclaimer, but these are issues to be aware of if you are looking for an alternate WebDAV provider.)

In any case, once you have a MobileMe account, launch OmniFocus on your phone, go to the Settings page and scroll down to the Synchronization section. Choose the MobileMe option, and below that enter your MobileMe user name. Then tap on the sync control in the center of the toolbar at the bottom. You'll get prompted for your MobileMe password (assuming your network connection is working), and if you enter that correctly (and you entered the correct MobileMe user name, and your network connection is working and MobileMe is working at the moment) then the sync should take place and future syncs should work without needing to prompt you again for your password or anything.