I have been having the same problem - just very inconsistenly - two iPhones, one attached to Mac and the other to Windows updated just fine yesterday - today the iPad connected to the same Mac keeps saying corrupted - it is sayingonly 3 minutes to download - compared to about 10 or 12 minutes for the iPhones yesterday.
I restarted iTunes - and the download is now showing more like 10 or 12 minutes total. and then it worked. or at least it looked like it was going to - got the full 700MB or so download and THEN said it was corrupted.
this also happened when iOS 5 first came out - I just kept trying - might have rebooted my cable modem - but not really sure that anythign I did or did not do had any impact on it.
You would think that if the issue was on Apple's end that they would have some wya to detect multiple failed attempts to download and look into any network issues on their end that might be affecting this.
Download of iPod update on another mac on the same netwokr is saying 30 minutes to download - which seems odd since the file sisze is the same - but for some reason that download is only running at about 2Mb/sec comared to 10Mb/sec for the iPhone and iPad downloads on this Mac.