The Zune forums are alive with complaints that a bunch of music has suddenly disappeared from Zune devices. The reason: it appears that Microsoft has pulled a bunch of music from the Zune Marketplace. If users downloaded this music to their devices via the subscription-based Zune Pass service, then it apparently disappeared when they synced their device. (I'm talking about subscription-based "tethered" downloads, of which users get an unlimited number per month. The Zune Pass also offers 10 permanent MP3 downloads per month, and those downloads have not been affected.)
I'm not a Zune Pass subscriber, but I did a quick check of the Zune Marketplace on my PC this morning and was able to confirm that a lot of music is no longer available, including songs and albums from high-profile artists like the Arcade Fire, Vampire Weekend, and REM (which still has some albums available, but is missing hit records like "Green" and "Out of Time"). If you want a more comprehensive list of what's missing, scroll through this thread on the Zune forums. A quick spot check of Amazon's MP3 Store and Apple's iTunes Store revealed that these albums are still available for sale there, so it's not like content owners have suddenly pulled them from online distribution.
I've asked Microsoft if it's aware of the problem and what caused it, and I'll update this post as soon as I hear back. My guess: some contracts with copyright owners expired or were up for renegotiation, and Microsoft hasn't yet renewed them. This seems awfully shortsighted given that the Zune software and Zune Marketplace are expected to be components of the consumer version of Windows Mobile 7 that Microsoft is slated to announce later this month at Mobile World Congress. But perhaps it's just a temporary glitch.
Update 3:30 p.m. PST: Here's Microsoft's official response: "We are aware that some users may be experiencing issues accessing a specific artist's content and are working to ensure users have the best experiences on our service. While we do not discuss the details of our business deals, we have ongoing negotiations with labels and content owners and are constantly adding content to the Zune Marketplace catalog."
That's not much information, but it sounds like contract negotiations are indeed behind the problem.