I’m still not certain exactly why Apple felt it necessary to release Safari for Windows, but if this experiment works, it will be good for Mac users; if Safari is used by more people, then there should eventually be fewer sites that won’t work with Safari.
I’m curious myself. Clearly we’re missing a piece of the puzzle. There was a business case for porting iTunes; there is almost certainly one for porting Safari. Compatibility and market share aren’t enough: I bet that Apple would have spent fewer resources if it had just directly improved compatibility. My guess is that Apple has got something up its corporate sleeve that requires a cross-platform Safari. They’re up to something. We just don’t know what that is — yet.
Update, 6/12: In his WWDC post, John Gruber points out some reasons for porting Safari: iPhone development and Mac evangelism are secondary, but the big one, he argues, is the ad revenue from the Google search bar, which already yields something like $2 million a month for Apple. Search engine revenue makes web browsers self-sustaining: it’s how Firefox raises funds. Google feeds an awful lot of people.