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.