I just received a heads-up from Microsoft that all of the Zune-branded services (Zune Pass, Zune Marketplace, Zune.net, and the Xbox Zune video store) are scheduled to go temporarily offline this upcoming Monday night (January 25) at 10 p.m. PT. The outage should last around 24 hours, during which time Xbox and Zune users will be unable to browse, stream, or purchase content from Zune Marketplace, Xbox video store, or access Zune.net. Existing content you've downloaded will not be affected, so if you're planning on a raging party Monday night, you should stock up on tunes now.
Once Zune services back online, Zune owners will be prompted to update their PC software. If you have an older Zune (anything that's not a Zune HD), you'll also be required to update your device firmware.
If you're thinking all these updates must mean that something big is on the horizon, don't get your hopes up. Microsoft is specifically saying that the downtime is focused on actual maintenance. The wholeisn't happening until spring.
That said, I'll be interested to see if Microsoft does make any sneaky little changes to the hardware or software. If you guys spy anything new after they flip the switch back on, please drop a note in the comments section.