YES!! I just installed Mountain Lion and discovered this exact issue in Safari 6.0.
I've noticed random jpeg images in different web pages are darker than other jpeg images which were saved with the exact same RGB color profile (both in Photoshop prior to Saving as JPEG for the Web, and when Saving for the Web (as a JPEG).
I discovered that THE PROBLEM GOES AWAY WHEN I revert to the iMac Color Profile, and view the webpage once again (as when I read the other posting regarding monitor color profiles I decided to check this out--I had been using a custom profile when I initially discovered the issue). The images appear to "re-merge" back into the surrounding other flowing background images. They appear seamless, side-by-side again amongst each other.
The colors of only SOME ramdom jpegs were off (when not using the iMac Color Profile) from other surrounding ones (again, ones which had been saved with the exact same color profile).
BUT HERE'S A FUNNY THING: After verifying the color correction between various jpegs in setting the calibration (back) to the iMac calibration, when I returned to check out my other various sets of calibrations, the two jpegs which had previously been "off" (darker), they were now color matching the other surrounding background images.
However, I had one small Adobe Flash image in the midst of these (which used to appear seamless in its color with the rest--when viewed in Safari 5.1.7)--it is now the only image which is no longer color matched to the rest of the page background as it used to be (unless when viewed in the iMac Color Profile, which sets it "straight" again).
Now why was I having trouble for the last three hours with all other color profiles, until I switched profiles to the iMac profile and then back again to the others?
There must be some other color profile information contained in the Flash file that doesn't allow it to shift around as have the other jpeg images.
Please . . . Apple: Take care of this issue. I, along with a lot of others, have been spending long needless hours trying to figure out what's wrong with this picture.