I haven't used an iPad Pro with the Smart Keyboard hands on, but from what I've seen in advertising based on the way it folds over to stand up and stay connected, you probably couldn't lay it totally flat like you could the Surface Pro line. Maybe a future version would allow it to stand flat if you wanted to, or you could just use it in the default standing position?
Of course, I could see using a Bluetooth keyboard and having the iPP to the left or right as a trackpad depending on one's laterality.
I feel like the iPad Pro would be too big to use comfortably as a trackpad on most desks. If you lay it flat, it takes up too much space. If you use it standing up, you have the "gorilla arm" problem.