T-Mobile's UK wireless data service has gone down, with no connectivity for Internet or other data services - as far as I can tell (I'm a T-Mobile subscriber: my phone still has its IP address, but I can't ping any other network addresses).
A call to customer support reveals that this is a nationwide problem and this may continue for a few hours. Other T-Mobile subscribers have confirmed that the problem is extant at least as far as Southend.
I've put a call into T-Mobile's PR, and they're getting back to me. Twitterers can put in reports or comment to #tmobfail.