once Apple makes the UI guidelines and hardware parameters clear, OS X developers will be free to port their apps. I would guess that that process won't be that much a barrier. However, I also believe a 10" tablet will be somewhat hardware constrained in such a way that certain marquee apps such as Photoshop or FCP just aren't going to be able to run well enough, so they won't be allowed to run at all.
I imagine the next iPhone SDK and Xcode will focus even more on aiding porting - from iPhoneOS or MacOSX - to the new interface & related technologies.
And then I suppose Apple will reject apps that don't move fully to their new Touch interface guidelines, until they fit a very specific look and feel that Apple has in mind. That would cause complaints, but could also be the key to redefining the way things are done.
And if you spray too much paint, it runs down the screen (it can use the accelerometer to work out which way is down). So when you paint if the wet paint starts running down the window you need to put the tablet flat until it dries?