Hi there,
This problem has been the bane of my existence since doing video work on a Mac. (PPro CS5, Mac OS X 10.6.8) So, whenever I export my project, regardless of container or codec, there is always a measurable contrast/gamma shift in the video file.
I am aware of the dreaded "Quicktime Gamma Bug" and I've spent a lot of time researching the "solutions" (x264 encoder, etc.) - none of them have worked. Some containers or codecs get the file closer to what it looks like in PPro, but not identical. When these files are uploaded to YouTube/Vimeo and view on a PC, the problem is VERY noticeable. Footage looks a stop or two darker, with a huge loss in contrast. I notice that the more I grade footage, the bigger the problem is once uploaded online (footage left pretty much alone stays relatively close to original). I have calibrated my Mac's monitor several times, and confirmed that the display gamma is set to 2.2. I should mention that this problem doesn't exist when editing on my home PC, just on my work Mac.
Again, I've done a lot of research into the Quicktime Gamma Bug, and I don't think it's that. (My most recent project was exported as FLV with a VP6 codec - when I uploaded to YouTube and viewed on a PC, still had the same issue.) So, anyone have any ideas on solving this issue? Or should I just toss my work Mac out the window and get a PC? 
Your tips are most appreciated!