I noticed that when I uploaded a recent photo to Flickr that the colours went all funny. They were not as rich or deep as they looked in Photoshop or iPhoto. Turns out that the problem was with Firefox which doesn't yet support colour profiling. I upgraded to Firefox 3, which I read did support this function only to find out that it ships without this support by default...why? Anyway there is a plugin available for Firefox 3, and on installing it all looks much better. However it is annoying that most of those people using Firefox will not be seeing my photographs how I want them to see them.
Here's a nice blog explaining it all by dria.org