You can reset the keychain for OmniFocus on your iPhone by visiting this link:
http://people.omnigroup.com/kc/Debug...tKeychain.html
You can get the settings restored by going to
http://sync.omnigroup.com and logging in. On the Account status page, there's a big Send to OmniFocus button that you can click to open OmniFocus and install the sync settings (assuming you haven't stored your database in a non-standard place).
You need to do both of these operations from the device that is having trouble! After you do so, tapping the Sync button should prompt you for username and password (which you'll have proven you know by logging into the OSS web page) and hopefully your iPhone will sync successfully.
Are you updated to 1.8.2 on your iPhone? It just became available today, despite what Brian wrote (confusingly, OmniFocus for Mac was already at 1.8.2). I make no suggestion either way about whether updating the iPhone will cure the issue for you; I'm just trying to get a clearer picture. Always a bit murky when someone jumps into a thread and says they are having the same problem though the problem reports differ :-)