It used to be that the beta 2 SDK couldn't run on the same computer as any other Kinect hacking software. Is this still true? Can I incorporate OpenNI and the SDK in the same project now?
That's a tricky one. Theoretically yes. The practical problem, however, is that when working with OpenNI, you need an OpenNI-compliant driver. The usual choice for that is SensorKinect. The Kinect for Windows SDK requires Microsoft's own driver, which is incompatible with OpenNI. Having two drivers operate at the same time is not possible.
So, Kinect for Windows SDK and OpenNI are mutually exclusive. Exchanging the driver is necessary when switching between libraries.
One possible way to make the SDK and OpenNI work at the same time would require you to write a OpenNI-compliant sensor module, which uses Microsoft's Kinect driver. Nobody has done this before, to my knowledge, and I'm not entirely sure that it would work. Check the OpenNI Programmer Guide for more information on OpenNI's architecture, if you intend to pursue this path.