If my perspective is correct, look for the Surface Pro to cannibalize sales from MS's "partners." I do expect it to appeal to the kind of person who buys a PC laptop and an iPad. And that's a great market for MS to defend. But for people who want just a tablet, I expect iPads to carry on selling by the container.
If Microsoft's vision of everyone needing a computer is correct, the Surface pro will destroy the laptop. But if Apple's vision of a "post-PC" world is correct, the vast majority of the tablet market will be just tablets, and the iPad will continue to thrive.
It all seems to come down to whether you think of tablets as weak computers with a tablet interface or whether you think of them as an appliances that run software.