Same problem here. Any enlightenment yet? I have restored iPhone to no avail and already trawled the iPhone forums so assuming it's a Vodafone problem.
Juzzie's issue was the SIM, so we'll work with you to find out what's going on with yours and get it sorted as soon as possible!
We'll need to isolate the issue, have you tried your SIM in another phone? If you don't have service in another phone then it could be the SIM or network. We'll have a look at your area, what's your suburb and postcode?
If you do, then it could be something with the iPhone. If you've done a factory reset then pop it into your nearest store and we'll assess your iPhone.
Let us know how you go and we'll take the next steps from there.