If you are looking at your phone and it says "NO SERVICE", make sure you're not waiting in vain.
It was consistently noted that once G1 loses the cell network, it takes it forever, if ever, to reconnect - one experiment lasted several hours, it never reconnected.
The cure for that is pretty simple: Settings, Wireless Controls, Mobile Networks, Operator Selection, wait until the scan ends, select your provider, voila, back in service.
Another interesting bit is that not all G1s are created equal: a black one loses the signal consistently in certain locations, white one stays on. I'm not saying the white one has a better antenna, may be just a coincidence - but there's about a month in manufacturing date between them, may be a hardware improvement.