Some labels have told Microsoft that it's not allowed to enable "squirting" for their tracks when sold through the Zune store. Squirting is Zune's much-touted ability to send your friends DRM-crippled tracks that can be played three times (or saved for three days) before they're erased. Microsoft won't tell you whether or not a track you're about to buy is extra-crippled -- nice deal.
Curious about this, I conducted a test of my own. I pulled down the top 50 songs downloaded from Zune Marketplace, using my Zune Pass subscription. I then created a playlist of those 50 songs, and attempted to wirelessly send the whole playlist to my wife's Zune.Link (Via Deep Links)
When the transfer completed, a message appeared on my player: "Can't send some songs because of rights restrictions. 29 of 50 songs sent to Carrie's Zune".