Because my broadband wasn't working I've been using internet tethering with my iPhone for my main internet connection for my computer. So when my iPhone demanded I update its software to cure its SMS vulnerability I thought hey why not go ahead! I figured it would download the update to the computer (via the iPhone's net connection) as per normal, then perform the update. No reason to suspect a problem. 

Unfortunately once it had got the whole 300MB package and tried to install it, it required access to some Apple server in order to finish the install, but the iPhone was half-updated and no longer providing internet connectivity. So I got stuck there, with an iPhone just showing a "connect to iTunes" graphic when switched on. I understand this is known as "recovery mode". And that's how it's been for a full day, but thankfully I now have broadband working and have managed to get it back up on its feet. That required a factory restore then update from backup then full sync though, and I'm yet to determine if all is truly well.

So the lesson here is – don't use your iPhone as the sole internet connection in order to update your iPhone!