I believe that the Apple servers are extremely busy today - with all of the new iPad, iPhone and iPod Touch activations and everybody is trying to download content right now. You may have to finish the download in iTunes on your computer and then sync the app to the iPad.
If you don't have access to a computer - try restarting the iPad and see if the download will resume.
Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.