Not trying to sound like an -hole or nothing like that, but were talking about two different kinds of devices and two different services. I think your correct when you say about the ipod blowing up once it was released to PC's. The PC market was and still is many times bigger than the Mac computer market will ever be. Itunes is a music buying service where as Zune is a music borrowing/streaming service. There is a Itunes software for PC's and there is Zune software for Mac. Thats as far as their sharing goes. There is no android, Windows Phone 7 or blackberry app/software coming from Apple and the same goes for Microsoft and it competitors.
Actually it's not really that different. The zune service also sells music and syncs devices and manages your media, same as itunes. It just has some additional options. Both services allow syncing and management of phones.
While OSX does have a WP7 connector software it really doesn't have zune software. Releasing zune for OSX wouldn't be a big gain on the desktop as mac marketshare is so small. It's all about putting it on the other mobile devices, ios and android. Microsoft could very easily expand their zune service and thus, revenue. It also opens peoples eyes to WP7 and what is possible there.
It's really win win for microsoft. As it stands now most people don't even know what a windows phone is let alone zune.
I don't think it's necessarily a long shot. Microsoft is porting it's lync software to android and ios. They also have a very nice bing app on ios that gets very good reviews.