It's the curse of gamma. Eons ago Apple chose a perfectly reasonable value for this obscure metric of color image contrast. Then Microsoft opted for very cheap displays that had a different value.
We know how that turned out.
In theory, I think (the topic is esoteric), colorsync software should manage the gamma effect. Unfortunately Windows does a terrible job with color management. So the problem remains.
With its next OS, Apple has throw in the towel ...
AppleInsider | Snow Leopard to see HFS compression, default gamma switchToday if you use a Gamma of 2.2 you can make your images look good, but the rest of the OS looks nasty. With 10.6 the OS will look good with a gamma of 2.2.
...Macs to date have typically employed a lower-contrast but lighter 1.8 gamma level, but the new Snow Leopard build now changes this to a deeper 2.2 gamma that was previously only an option in earlier Mac OS X editions...
I suspect our old photos will look good in 10.6 as long as they have color profiles embedded, but going forward photos will look good even on color-damaged Windows clients.
So this is actually a good thing, though it's also an admission of defeat.