Well it IS possible. BUT, it's a long way to go.
Here's what you do.
Boot up your vista -
Go to "My Documents" - "My Music" -
Now copy the entire "iTunes" folder to your external harddisk (preferably firewire) -
Also copy all the songs if they're not in the iTunes folder -
Shut down your Vista
Turn on your mac -
Run iTunes and clean it. Delete anything that's in it. Then hit command+Q (Quit).
Plug in your harddisk and copy the "iTunes" folder to your user folder - Music - iTunes (Replace the old one if there's any) -
Now run the iTunes application again -
Go to file - import (or shift+command+O) -
Look for the .itl file (or .xml) in your itunes folder - Open it -
Here comes the hardest part
Since the file system on Mac is completely different, you'll have to locate every single song manually from your iTunes Music Library -
I don't think there's any other way to do it.
But there's something you can count on, even if you haven't fixed all the songs your iPod would still sync the data (how many times played and etc.), I know it's weird but trust me, it's true.
If you're a "history-freak" like me, then you'll think it's worth it too.
Bytheway, no one told me this method. I tried it myself and worked fine. I didn't lose a single song data from over the past three years and my iPod (Classic 80gb) is syncing fine from my new MacBook Pro. I don't use my iPhone to listen to music, so wouldn't know about that.
Good Luck,
Let me know if you have any questions.