Seems that AT&T decided to send me a new SIM for my phone, and deactivate the old one. Not that serious normally, except that I’m not at home at the moment, so I didn’t get the new SIM.
Worse, when I spoke to them, they confirmed that once the SIM is disabled it cannot be re-enabled, and even if a new one was shipped out to me here, it still wouldn’t help as they need to be activated the first time on the AT&T network. So, no cell phone for me.
Even more annoyingly, the N95 insists on telling me about this every time I return to the standby screen, even in offline mode! I would switch it off entirely, except that I use it as my camera, email and web browsing (over Wi-Fi) and as a VoIP telephone as well.