There are two tweaks that do this: MountainLionCenter for iPad (free) and MountainCenter ($2.99). I'm using the free one because I find it works better for me (for whatever reason, MountainCenter was buggy). As they do the same thing (in my opinion) - may I suggest you go with the free one...
Then, it'll be the same as if it was the center of the iPad. Just know that some of the Notification Center tweaks don't work well on the iPad. They are designed for the iPhone and so sometimes the layout is a bit wonky, regardless of whether you have the center dropdown or the side.
Anyway. Have fun!
Marilyn