I went to the Apple store and the first thing they asked was - which cable did you use? Is there a difference I asked? Well, technically not, but make sure you use the original iPad cable. Huh??
I went back home and tried it - it's like day and night. Much, much faster backups, movie transfers take 1/5th of the time it took before.
So I thought that maybe my cable was damaged? There are three iPhones in my household (1 x 4G and 2 x 3GS), I tried all three iPhone cables (one of them is brand new) and ALL 3 were much slower when it comes to backing up the iPad. If I skip the backup and only sync then it takes about the same amount of time as with the iPad cable).
Then I called a friend who had similar problems earlier that week and asked her to try syncing with the original iPad cable - she called me later that night saying the difference was very noticeable.
So YES, there's a difference between the cables. I marked the iPad cable but you can tell the difference by the edges of the USB plug in, they're rounder versus the more squarish iPhone USB connector.
Later I figured I'd google this problem and found out I wasn't the only one with this problem. I'm not allowed to post links yet but there are several apple disussion forums I found where they found the same problem I did. Try h t t p:// in front of discussions.apple.com/message.jspa?messageID=11570395 Same issue, same results...