I have a iPad 2, I updated to iOS 5 and I have added the Mail app to the Notification Center in the setting. I have 4 emails and when I drag down the Notification Center from top it say "No Notifications" 🙁
since email is often very private, it is by default left out of notification center... you need to go to settings > notifications > mail to change it to "on" and make sure "show preview" is activated.
if that doesnt work try restarting the device and sending yourself a new email.