Phew! scary couple of minutes
Apparently, when you sync up the contacts on an android phone with the internet, the protocol involves deleting every single contact on the phone and replacing it. If your data-connection gets interrupted half-way through this leaves you with an empty contacts list for about 5 minutes.
Needless to say, I had no idea what was going on, or why I'd lost all my contacts. Wasn't happy with android. I'm somewhat mollified now they've all (hopefully) reappeared.