Bruce released 3 very informative video on his website about how you can control the exposure of the Canon 5DMarkII using neutral density filters. The principle is simple: if there is too much light, the camera will close the aperture, which in turn will increase the depth of field. The solution is to use the filters to reduce the amount of light that goes into the camera, so it opens up (automatically) it aperture, and create a nice bokeh in the process.
Video 1: shows the various filters and explanations
Video 2: explain the advantages of 4×6 filters vs 4×4
Video 3: example of the video captured
Over all the whole thing last about 20 minutes and it is a good investment if you want to shoot video with your 5D. Go and watch it! My only complain is that Bruce did not show the one thing I was interested to see: Depth of field changes controlled by the circular variable ND filter.
Note: you can also use a circular “normal” polarizer to do the job, it is just that the variations are between 0.5 and 2 stops so you will only see much of the difference if you are in medium lights.