I have unfortunately had this happen to me a few times, which got me thinking about sharing with the world. That is, the dreaded situation if your iPad or iPhone app is stuck on 'waiting', when you select update in the App Store.
This is most notably when I upgraded to iOS 6, and in particular on my iPad. It then stays stuck and won't update, which is quite annoying because it means you can't even use the app (if it's something like facebook which is used all the time).
- Sync your iPad or iPhone with iTunes on your computer and then sync to transfer the app to your device.
- Go to Settings, then iTunes & App Stores, tap your Apple ID to sign out, restart your device and then go back and sign in to the iTunes App Store in Settings Again.
- Single tap the app that is stuck in waiting, to toggle it to paused, then go to App Store and select update for that app, which will force it to re-update.