According to OpenNi Nite's own licencing terms you're not allowed to bundle NITE or sell it with your software.
For further limiting factors, OpenNi is Apache 2.0 licenced (see specs here)
So in summary, it's possible to use OpenNI / NITE to write a commercial application, but because you are not allowed to bundle NITE, it becomes harder to distribute than it would be with Microsoft's official API.