It's seamless 2G roaming...
They've finally got round to getting seamless roaming for 2G.
From Digital Spy (plymouthbloke1974), the rollout dates are:
12th Oct: Gloucester and Midlands area
W/C 31st Oct: London and Wiltshire area, and down to Brighton area. North Wales, north west England and Scottish Central Belt.
W/C 7th Nov: Norfolk, NE England and Aberdeen area
W/C 14th Nov: Everywhere else.
3G roaming is expected to follow a similar rollout pattern, but will still drop the network when switching over.