Be aware that the fix Jordan posted stopped working for me a while ago. Yes, the bug is with Apple's h.264 codec assuming 1.8 gamma but displaying 2.2 (or the other way around, I forget). If you go into your Mac's display preferences you can select which gamma you want and that might get around it. For some reason the Apple codec seems to determine which gamma is currently set and then embeds that in the video, but your results may vary. You can get around it by using x.264 as the encoder, but it might take a lot more tweaking with that codec to get a good quality/filesize balance. Also, I've had some stability problems if using compressor on a cluster with x.264 which I've never had with the Apple one.