A bit more useful than I thought it would be.
Pretty simple really I set up the D90 for Auto ISO (ie pick a max ISO limit I want) set the camera to M mode. Adjusting the aperture/shutter speed the camera then sets the ISO level to the "metered" exposure. So reducing the exposure ie faster shutter speed or smaller aperture and the ISO value climbs up to the max ISO you have set in camera.
Pretty similar to the TAv mode in the K-5 (but I didn't have it on the models I had)
If you want manual ISO simply disable Auto ISO in the menu.
In a nutshell M mode with Auto ISO enabled is like having shutter and aperture priority in one mode.