Not sure if this is possible...but I'm sure anythings possible with the right programmer. There should be an app to link your iPad and iPhone over Bluetooth and have it link your SMS from your iPhone to your iPad. When connected and you received a SMS on your iPhone the notification would pop up on the iPad if the iPad was in use. If it was sleeping it would go to the iPhone. Any message sent using the iPad would also need to show up in the SMS application on the iPhone as well (that would just be annoying if it didn't).
Not sure how hard this would be to make or if it's possible...
Not sure how hard this would be to make or if it's possible...