Despite some glitches at Nokia's end, telling me the N78 was already up to date and that the firmware version available was the same as the current one, v12 - hey, I 'updated anyway'. And lo and behold the N78 comes up with v20.149 firmware after all. Something screwy obviously present there, but worth a good look over-the-air or via Nokia Software Update if you own an N78. Do a backup anyway, but in theory all your data and apps should be safe.

See here for a detailed changelog.