[Andrew]

If you don't have any changes on the phone which you need to keep, and the deletion hasn't synced to the desktop yet, you can reset the sync database from the desktop's database, then sync again on the phone to get the restored sync database on the phone.

If it _has_ synced to the desktop already, you can open one of your OmniFocus backups (assuming you have an up-to-date backup), find the the project in question there, and drag it back to the main OmniFocus database. (Then syncs all around in order to push the project back to the sync database and the iPhone.)