Being an Über-Geek, I of course own an iPhone. This morning was therefore upgrade time for me, to firmware v2.0. Everything went pretty smoothly, save for this one nonsense error message:
Cancelling and retrying made the problem go away (why do computers always defy the most fundamental principle of physics?). You would think that -1.48GB would still evaluate to less than 12.06GB, and so that the backup should have been carried out anyway.