Correction - I just tried a fourth time and it worked. I let the iPod power off, then turned it on and launched iTunes before connecting the iPod to my computer. I don't know if that's what made the difference, but it did work this time.
Restarting iTunes did the trick for me; I left my iPod (a wheezing old 60gB video version) plugged in, closed iTunes, then relaunched it and presto, no more unknown error 1433, but an updated iPod.
I had the same error 1433 when I tried to update my old 30 gig ipod. I followed "mudgeek" advice, and everything's ok now, ie plug the ipod in, then start itunes.