Oh dear, O2's massive outage is rolling over into its second day and there's currently no definite timescale for a full fix. Apparently the issue is being caused by a central network fault, which prevents people's phone numbers registering on the network. O2's managed to restore its 2G network, so if you're on O2, disable your 3G for the time being. Slow data is better than no data.
It seems to be random who actually gets hit by the outage, and it's causing just as much of an issue for any MVNOs on O2, including GiffGaff and Tesco Mobile. O2's working hard to get things fixed, as you would seriously hope, with engineers powering through the night. Apparently now that O2's 2G network is back up, its 3G network is slowly coming back too. Considering the cock-up has hit customers across the country in their 100,000s it's just a tad embarrassing for O2.
O2's current advice for you, apart from turning 3G off, is to keep switching your phone on and off throughout the day in an attempt to properly register your phone on the network again -- keep flicking that airplane mode switch folks, hopefully the data will start flowing again soon.
Update: For all those who like things spoonfed to them via video, here's our very own Kat dropping some knowledge on the BBC this morning:
Update 2: According to O2, things should be back to normal now -- so try reconnecting to 3G and let us know how you get on.