iMessage activation on iPad - unable to verify email

I have just upgraded iOS on both my iPhone 4 and iPad 1. iMessage on the iPhone is working fine. When I try to activate it on the iPad, I've been getting the error message: "Unable to verify email because it is already in use."

When I first started the iPad after upgrading the iOS, I entered the Apple ID/password combo that I have used for my iPhone (which is the same as my MobileMe ID/PW). This is the only ID/PW that I have been using with Apple. I was under the impression that you could use the same email address for both devices.

Would someone kindly advise. How can I set up iMessage on the iPad? What am I doing wrong?

Thanks!