Comment 20 for bug 631426

all you need to do is change the server. it worked for me ^_^ already upgrade from 10.04 to 10.10 just now.

go to System > Administration > Update Manager > Settings > Ubuntu Software (tab) > Download from :

then change it to "Others". it will ping for the nearest and fastest server near you. or you can manually choose it if you want.
and then upgrade process run without problems :)