Despite my hopes yesterday, I didn’t get a chance to try sharing music between Mac and Windows iTunes last night, but I did copy a few songs I bought from the iTunes Music Store using my Mac to my Windows machine this morning. When I asked it to play the first track, it prompted for my music store account information, connected to authorize this machine, and then played that track and the others that were brought over in the same way.
So the downside is that moving tracks from one machine to another is not seamless, requiring you to dig up the files and physically copy them over; the upside is that once they are there, authorizing the other computer is painless.