It works that way for Apps, and while I'm sure you can play music and movies on iTunes logged in with the same account, I don't know about re-downloading them. Some people have had issues (the media companies are complete biatches about that).
Of course, you can simply transfer the file between machines, which is what I do.
Just drag and drop into another iTunes instance logged in with the same creds, and you're good to go.
11-26-2008 04:18 PM