I had tried to download iOS 4.21. update for my iPad since today morning and it had failed 8 times in a row. This is most probably happening because of slow internet connection at my home and apple servers are already going nuts for sure.
However according to this thread of Apple support you can download the update for iOS 4.2 for iPad from here ( http://appldnld.apple.com/iPad/061-9857.20101122.VGthy/iPad1,1_4.2.1_8C148_Restore.ipsw ). Best part is that it you can pause and resume back anytime.
Since the link is from the download section of Apple, I am sure its pretty safe.
Update : There is WiFi issue with iOS 4.2 but a fix is available, Make sure you read about it here.