With web viewing the way colors are interpreted by the browser can have a major effect on how an image is seen. That's why the "safest" color space to use for web posting is sRGB.

If, for example, I forget to convert from ProPhotoRGB to sRGB the colors will look like crap when presented on the web to most people. The people that have browsers capable of reading the assigned color space will show the photo more accurately.

Safari has been the only color managed browser for a long time, but Firefox just came out a color managed version. I'll run down and post a before / after to better illustrate what I'm trying to say in a bit.

Best,
Jim