The service is back at around 18:15, having gone down some time before 15:45. Still no news from T-Mobile as to what happened, but a quick check of Internet access and various apps on the G1 shows it's running at full speed.
Incidentally, the burst of data that kicks off when all those apps finally get their connectivity back goes on for some time. It seems, unsurprisingly, that software designers have the same mindset for mobile apps as they do for the desktop - the Net's there, and it's there to be used as much as necessary. Not such a problem for all-you-can-eat data tariffs, but that does have implications for battery life - and by jiminy, don't turn on data roaming when you're abroad.
UPDATE - T-Mobile got back to me with impressive efficiency. The official statement is that mobile broadband and Internet connectivity were both down from 15:15 to 18;20, but they are now fully restored. They can't say exactly what the issue was, but the spokesman said issues with the DNS servers were suspected.