Oh dear. Just when I'd got all excited again about Zune, on the premise that only DRM-encrypted audio would be subject to the draconian 3 days/3 plays policy, one of Zune's PR contacts got in touch to let us know that, in fact, all audio will expire according to those limits. If you were thinking you could just re-transfer it over and over, then you'll be pleased to hear that Microsoft has also put a stop to that, too - apparently multiple consecutive song swaps are limited as well.
If I were a PR person I think this is the part of the job that I'd enjoy the least - having to put people off buying the product I represent by justifying its crippling limitations as insisted on by the manufacturer (or the manufacturer's powerful content allies). Microsoft can warble all they like about song metadata remaining on the player and the music staying in your list of files, but consumers are going to see WiFi and think "great, wireless transfers!" and not "great, self-destruct wireless transfers." I'm afraid Zune has just fallen off my shopping list again.
[Thanks to Sara @ Edelman for the info]