[miketoll]

If it stays on the mirror and does not transfer to the sensor then it has no effect other than being annoying. I had this problem with my film SLR and in my ignorance just wiped the marks off with a lens cleaning cloth with no harm done. I would be a little more gentle now but if it annoys (which it would me) then I see no reason not to clean the mirror like a sensor. Marks can also become apparant on the screen that is above the mirror that the mirror comes up against when it flips up during an exposure. These marks will be more in focus than the ones on the mirror itself and are more likely I would have thought to be the ones you are seeing. Again they will not effect the photo and just carefully remove.