Are there sites that support editing and ordering photo prints, cards, and books on an iPad?
I don't really have a direct answer for you, but I would like to help by sharing what I know.

I have a Mobile Me account, and there are no tools for editing your photos online. It's an image gallery. A gallery is meant just for viewing and not for editing.

Mobile Me was designed to work in conjunction with the iLife software on a desktop mac. You edit the photos, order the prints, make the cards, and the books in the iPhoto app on the Mac.

Photo editing on the iPad is the easy part, but I think you are approaching it from the wrong angle. There are tons of free (and cheap) apps that allow you to edit photos. (Photoshop Mobile for instance. Just check the Photography category in the App Store). Once you edit them in an app, then I would upload them to a service that supports printing services. The hard part is going to be the ordering of prints, cards and books from the iPad.

We can only hope that sometime in the near future, Apple creates a mobile version of iPhoto just as they did with iMovie. That would be ideal.

EDIT


You can order prints from this app as well:
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=323705153&amp;mt=8%3FpartnerId%3D30&amp;siteID=KEmRFwU0WKY-yY5rDqaKJMaSUWCfbLPC3w
You can also order prints from the Walgreens app:
http://itunes.apple.com/us/app/walgreens/id335364882?mt=8
This might solve your issue as well!
http://www.ritzpix.com/net/OrderPrints/
You might want to check out this app and service:
http://www.ecce-terram.com/order-solutions/mobile-client.html



  ECCE TERRAM announced a new service this week that will allow people to order prints of photos stored on their iPhones from the phones themselves. With their Photo2lab Client, users may select a photo (both those taken with the iPhone and those transfered from iPhoto), add text and create prints, post cards and even mini photo books. You'll even be able to add an address to a post card from the iPhone's built-in contacts list.