Here's how I finally removed the preferences pass code on my iPhone 5 with iOS 6. Going into plist doesn't help any more. You can turn off enable restrictions but you can't see the pass code you forgot. It now says "BLOCKED".
You will need 2 phones having the same OS (I think). I used an old iPhone 4. A 3gs also worked but it didn't have enough memory to hold what was on the 5. I don't know if you can use 2 of the same model. probably yes. I also had to go into iTunes and unselect some stuff to sync so it would fit on the 4. That was easy to sync back on at the end.
Here's how I did it:
Backup the iPhone 5 with iTunes.
Backup an old iPhone 4 with iTunes (or any other phone having the same OS).
Restore the iPhone 4 to factory settings (may be unnecessary). It will have no pass codes.
Restore the iPhone 5 backup to the iPhone 4. The forgotten pass code will not come with it but everything else on your iPhone 5 will be there, including email, text messages, apps, folders, app data. No passcodes.
Backup the iPhone 4 with iTunes. This is the backup you will restore to the iPhone 5.
Reset the iPhone 5 to factory settings. To do this, press the top button together with the big front button until the phone turns off. When it turns off quickly let go of the buttons then hold the big button down while connecting it to the cable which connects to your computer. Have iTunes open. It will prompt you to restore to factory settings. The passcode will be gone.
Restore the backup from the last iPhone 4 backup. Your phone will be back to normal without the passcode!
If you had to unsync some stuff in iTunes, sync it back on.
Make a new backup so you have one with no pass codes.