"This device isn't eligible for the requested build"
I'm trying to update the OS on my iPod Touch. It's model PC086LL, currently running iOS 4.0, and iTunes says 4.2 (or maybe 4.2.1) is available. So I tell iTunes to update it, and I get the error message "This device isn't eligible for the requested build". The "More information" button takes me to http://support.apple.com/kb/TS1275 , from which the obvious link seems to be to http://support.apple.com/kb/TS3694 , which lists that error message but has no suggestions other than "update to the latest version of iTunes," which I've already done. (10.5.2, right?)
Any ideas?
iPod touch (2nd generation), iOS 4