The way Apple currently handles FaceTime and iMessage across multiple devices doesn’t work. An iMessage is sent to your iPhone’s number, and it doesn’t show up in the Messages app on your iPad or the beta Messages app on your Mac. The iPhone is the only device that can be reached via iMessage or FaceTime by both a phone number and Apple ID. The iPad, iPod touch and Mac all have to use an email address. If an iMessage is sent to your iPhone’s number, it doesn’t show up on your iPad. So you have to tell your friends to use your Apple ID address for iMessage if you want it to work on all of your devices.
This is a severely flawed way of handling communication between devices, but the good news is that Apple has fixed this problem in iOS 6.