Yes, this is an incredibly detailed title, but I couldn’t quite tweet the problem and solution so I’m doing a quick entry. When upgrading, like (hopefully) most people I make a backup.

It seems a new “feature” with this configuration (my guess is bug in iTunes) is that if you’re doing the backup, when a “Do you want to upgrade to iOS 7” panel pops up, no matter what you answer it will “stall” the backup, in that it will never complete (in my case beyond step 2 of 4).

The solution to this is quite simple: Start iTunes, but don’t start your backup yet! Just wait (a few mins?) for the “Do you want to upgrade to iOS 7” message to pop up, select “Later” and then backup your iPad.