All the cases I've seen for the iPad are basically the same size as the iPad, which means they're too small to have a 10-key. Even the keyboards on those "keyboard cases" have to make the keys a little smaller than normal due to the iPad's size. (Roughly 90-92% of normal.)
I know you said you didn't want a separate keyboard, but one of the few "portable" keyboards with 10-key I've seen is the Matias:
http://eshop.macsales.com/item/Matias/FK204/
I have not personally used that model so I don't know what the feel of the keys are, but I don't know of any more keyboard still being sold that have 10-key and are "portable". You could use any Bluetooth keyboard, but pretty much everything else with a 10-key is a not-so-portable desktop keyboard.
There is the LMP 10-key add on to the Apple Wireless Keyboard.
http://store.apple.com/us/product/H4606ZM/A/lmp-bluetooth-keypad?fnode=MTY1NDA1M g&s=topSellers
I have not tried it on the iPad, but if it does work, then the Apple keyboard could work for you. It would be a little larger than a "true portable keyboard", but that might not be a bad thing since too small of a keyboard would probably cause more typos for you.