Bit of a laugh this is.
Just called o2 because I still have 'No Service', nearly 24 hrs after registering it up with iTunes.
Seems like loads and loads of orders are being put through manually. She basically said that these manual entries will take 2-3 days to have their phones activated. Call back in 3 days if you dont have a service.
Quite amazing considering I ordered and gave details over the phone to CPW on the Thursday at approx 10:30am.
Not impressed by the roll out I must say. Total balls up imo.
Bit of a long shot this but, you might be in a non 3g area so try turning the 3g off in the menu and see if you get a signal.
It sounds daft I know but when I got mine where I live the 3g signal isn't the best so I was on 1 - 2 bars. Turning it off and going back to gsm I was on full. Could be whats causing the no signal?