It’s probably been reported extensively elsewhere, but today was the first time I’ve run into this issue. I was trying to lay out a page which contains quite a number of alpha-transparent PNGs, a few JPEGs and a variety of background colours, and one fade wasn’t working right. In Firefox and Internet Explorer, the background image faded neatly into the background colour. In Safari the background image seemed distinctly darker than anywhere else, and wouldn’t match up properly.
Most of the online resources I found made mention of some problems with PNG colour profiles, but there was little discussion of JPEGs. After a little while reminding myself where Photoshop stored such information (Image -> Mode -> Assign Profile) I was able to confirm that the image had been saved with an sRGB profile. Safari is the only browser that honours such information when rendering a page, so it was displaying the image correctly while the other browsers weren’t. Resaving the file without the colour profile seems to have done the trick.