Reports of resets, and then the Zune being frozen on startup, started surfacing overnight. You can find threads in several forums, here, here, and here. Note that all the forums are busy, and slooooow to load, at best.
I have to emphasize, this appears to only be occurring on the first-gen, 30GB Zune. It seems the issue started to happen as devices passed midnight locally across the world, though times don't seem to be exactly synchronized. One can imagine a "wave" frozen Zunes moving across the planet.
Z2K makes sense as a moniker for this: the theory that immediately came to my mind because of the seeming synchronization with midnight was that this was the first leap year since the 30GB Zune was introduced. Others have come up with that idea as well. It has also been reported, though unconfirmed, that older firmware does not have the issue.
There are troubleshooting links on Microsoft's Zune set, here, but I have to repeat: you can't reset the device, and it seems to be a firmware issue. It's probably best to wait for a Microsoft response.
However, due to the timezone differences, there's no official word from Microsoft yet on why Zunes with the newest firmware have frozen. Let's be honest though: the Zune has had enough problems trying to gain market share against the iPod. This can only hurt.
I'll update this as the day progresses. 'Til then, watch a frozen Zune trailer attached to this post.