A thoroughly disturbing post has appeared on the Android Guys website. It seems to indicate that it is not possible to turn off the internet connectivity on the T-Mobile G1 Android phone.
A US reader of theirs wrote in to say he had visited the UK with his G1, having thought he'd disabled all data connectivity. It looks like the phone just kept on pinging, though, and he got hit with more than $100 in data roaming charges.
I hope there's another explanation for this, and am waiting for T-Mobile to get back to me with their thoughts and some clarification as to whether UK handsets could be affected. If the case is as it appears, though, then going abroad with your G1 is a really, really bad idea.