I notice sometimes that meat in the display counter at my local butcher shop has a green and magenta sheen to it.  What causes this?  Is the meat still okay to eat?

--Toni

What you are noticing is likely a phenomenon referred to as iridescence, or more technically birefringence.  Essentially, this is the same effect as light passing through a crystal and splitting into a rainbow of colors, only the rainbow is from light reflecting off of a surface rather than passing through something.  This can happen with either fresh cut or cured meats, and depends mostly on the angle at which the muscle fiber happens to have been cut.

Birefringence is more noticeable on darker colored meats like beef that lighter ones like chicken because the dark background gives greater contrast.

Provided that it is caused by the reflection of light from the surface and not a permanent green color to the meat, it is still safe to eat.  You can tell by moving the light source, the meat or yourself to a different position and seeing if the rainbow colors shift or disappear.