Scarab:
Just checked it on my Orange iPhone 4 and it says “To enable this on your account please contact Orange”
I have a Jailbroken iPhone 3G but not interested in doing it to my 4, caused me no end of hassle when I needed to get it repaired! Did save me a fortune in apps though!
When you need it repaired you just do a restore in iTunes and your good to go.
When you get handset back you just jailbreak again 