It's not clear to me from the linked article, but it sounds like Microsoft want all apps running on the phone to be "managed" code running on the CLR engine. This is just plain sense. It means that they can then run all apps in the same memory-space, and be sure that they are all nicely "sandboxed" so they can't corrupt each other's memory. If non-sandboxed code is allowed, then the OS has to run each app in a separate process with its own memory-space. That makes life more complicated, and adds overhead. I'm sure microsoft don't want to handle user complaints about os crashes, and have to analyse them knowing that some of the processes on that system are not sandboxed. It's a little like the Linux "tainted kernel", where you get absolutely no support if you load non-gpl kernel modules. Of course in Linux, you have the *option* of loading the modules if you really really want to. Still, I'm sure linux distros aren't keen on having lots of users with tainted kernels, and that Microsoft feel the same about their phones. Limiting apps to using CLR isn't crippling, and with the XNA lib for fast graphics added things should be even better. Ok, not quite native speed but pretty close...