I've had this happen twice and fixed it twice. Here's been my experience:
If I unplug the iPad without ejecting it from iTunes, iTunes gets "stuck". When iTunes is stuck, it doesn't see the iPad when I connect it again later. Rebooting does not fix the issue, restoring the iPad does not fix the issue. There may be other events that cause iTunes to get "stuck" like this, but both times for me was due to unplugging without ejecting from iTunes.
In order to fix the issue, both times the only way I found to do it was to reinstall iTunes. To do this, drag iTunes to the trash, empty the trash, and download and install a new copy of iTunes from
http://apple.com/itunes/download
You don't need to download every time, but keep the install file handy because you may need to reinstall. I expect this issue to be addressed in upcoming versions of iTunes, it seems like a fairly simple oversight. It's common for developers to miss an error that only occurs when you do something you're "not supposed" to do (which is why we have anonymous user statistics).