We don't have any information on SL5's 3D API yet so it's all guesswork. But in the keynote they said that it's an immediate mode API and that it supports custom shaders. Those are the key things in my view.

XNA, OpenGL and WebGL are all immediate mode APIs so it could be like any of them. Making it like XNA would make sense so that if they ever bring it to WP7, it'll drop in easily for devs that already have XNA experience there. Likewise for XBox. On the other hand, making it like WebGL would be a plus from the point of view of web developers.

FWIW, the cross-platform issue is a bit of a red herring. The different 3Ds are not so different that they cannot be implemented in terms of each other. As an example, check out the opensource project called XNATouch. That implements the XNA 3D API on iOS running under MonoTouch, translating XNA calls into OpenGL ES calls as it goes.