I noticed sometimes my iPad takes FOREVER to sync. Specifically, my ipad takes forever to backup before it starts syncing. I recently stopped syncing it to my macbook pro and began syncing it on a windows 7 system - iTunes has been behaving awesome for me on Windows, contrary to frequent opinion. At first, I was careful, and only ever used my iPad cable... but it's just a USB cable, so eventually I slipped and starting using my iPhone cable.
I'm not sure what's special about the iPad cable, aside from combined with the iPad charger it is a higher power charger, but it appears that backing up your iPad (even when it has practically nothing on it - as I recently cleaned mine out) can take ages if you use the wrong cable. Facepalm. It took some reading up on this to realize what I as doing wrong. It's a double facepalm for this even being an issue - I am not sure why an iPhone 3G or 4 cable would cause slow syncing...but it looks like it does. In my case it's slow backing up...like 30 mins to an hour when nothing has even changed. Problem solved though - use the iPad cable.