Mosaic Engineering showed a filter that it said to remove lots of annoying issues with the Canon 5D MKII: The filter can be easily removed for standard photography and requires no modification of your hardware (which means no voiding of warranties).
* Optically corrects most aliasing and moiré artifacts in 5D Mark II HD video.
* No reduction of 5D Mark II 1080p video resolution for most lenses.
* True optical correction before video image capture – no postproduction software filters or processing.
* Easily installed or removed in less than 20 seconds.
* 5Dmk2 H.264 codec compresses with better quality with the VAF-5D2, because false, high-frequency, aliased image components are eliminated before compression.
Moire, caused by the downscaling from the image information from the huge 21MP chip affects by line skipping, manifests in issues on fine fabrics, hair, bricks, water, lines. This filter removes all that, and more.
PLUSSES: Removes most of the visible moire, only the most stubborn stuff it cannot remove. Big success on aliasing. A massive improvement on image quality for video compared to shooting without it.
NEGATIVES; Not good with wide angle lenses. Some lenses work better with it than others. Not parfocal. Slight image softening. Close up focus changed. Be careful with old Nikon lenses with the bit of metal that sticks out. It can damage your filter and if you are not careful like me…your camera too! (Update: this issues seem to be fixed in the production version.)
[Update 2012-03-11] I was asked if this filter can be used on the Canon EOS 5DMKIII and I actually think it can, while it remains to be tested if it actually makes sense to use it. The Canon 5D MK3 still has moire: its processor doesn’t line skip anymore but it still has to process the image down from 22.3 MP to 1920×1080 which may create some minor moire or aliasing issues.