I seem to have invented a new extreme sport on the way into work: Ice Cycling. The roads were like an ice-skating rink. Scary stuff :(
Here's some advice for anyone in the same boat:
use a high gear: avoid using low gear if possible, even when starting off. Low revs mean you're more likely to get traction.
try to avoid turns: keep the bike as upright as possible.
try to avoid braking: braking is very likely to start a skid in icy conditions.
use busy roads: where the ice has been melted by car traffic. In icy conditions, you should ride where the cars have been, since they'll have melted the ice.
ride away from the gutters: they're more likely to be iced over than the centre of a lane. Again, ride where the cars have been.
avoid road markings: it seems these were much icier than the other parts of the road; possibly because their high albedo meant the ice on them hadn't been melted by the sun yet. So look out for that.
Here's a good thread on cyclechat.co.uk, and don't miss icebike.org: 'Whether commuting to work, or just out for a romp in the woods, you arrive feeling very alive, refreshed, and surrounded with the aura of a cycling god. You will be looked upon with the smile of respect by friends and co-workers. - - - Or was that the sneer of derision...no matter, ICEBIKING is a blast!' o-kay.
Their recommendations are pretty sane, though. ;)