Hmm.. O2 down.
I just had to announce that on Friday night, in central Manchester I was without mobile for 4 hours with the o2 network down. i tried calling my Friends on o2 and couldn't get in touch either. Headed out for a night out and a few punters were complaining about there phones not working - til I told them o2 was down..
There we have it. (I'm assuming it's got something to do with them downgrading their network to work with EDGE).