Since Apple released iOS5 there have been a number of reports of people unable to log into their corporate Exchange accounts. There are several methods you can try to get your iPad, iPhone to connect to your Exchange server, this is one of the easiest to do and is often overlooked since Apple says in the Exchange set up on your device that entering the Domain name is optional. It is not. Enter it as you're entering everything else and you should connect fine. If not you'll want to try the other methods such as removing iCloud - resetting your network settings and then setting up Exchange.