The Zune HD is, as you'll know, Microsoft's answer to the iPod. As such, they put a fairly hefty amount of work into making it as impressive as can be; this, of course, includes firmware updates. The next Zune HD upgrade will bring XviD and Smart DJ support.
According to CNET, the update will be hitting sometime this Spring, and noted that whilst XviD support is in, DivX support is unlikely. On the bright side, the new format will support the HD AV dock accessory, though Microsoft wouldn't clarify the official resolutions supported. The other main feature is, of course, Smart DJ, currently only found in the Zune software for PC. When this feature debuts, it'll mean that if a user has a Zune Pass, and is on the same wireless network as the PC in question, then any Smart DJ playlists they create will include music streamed directly from the Zune Marketplace, as well as content already found on the device. A rather handy feature, to say the least.
As mentioned, the update will come sometime during Spring. Microsoft employees also reminded CNET that Facebook for the Zune HD will be arriving early this year, with the promised Twitter app already available on the device. If there are any other features due in the firmware update that are announced at a later date, you'll be able to read about them on Neowin.