From our Zune blog:
Wireless sync is one of the most exciting new Zune features. An industry first*, wireless sync allows Zune players to, well, sync wirelessly with your host PC.
Over your home wireless network, you can sync your Zune whenever you wish.
Your Zune player will also sync automatically whenever it's plugged in - either through a dock, an AC adapter, or a USB cable.
The convenience of wireless sync is even more evident when you consider Zune's new support for podcasts. Wireless sync will make it easier to keep your frequently-updated podcasts, as well as your playlists, pictures, and videos, synced with your PC.
What's more, this new wireless sync will be available not just for new Zune players, but for all of us who bought first-gen Zunes. The capability will be delivered through a firmware upgrade in November.
Thanks, Microsoft, for not leaving us early adopters behind! *Correction: Not quite a first: the Archos provides wireless sync in its Archos 604 player. Thanks, Palmer. - Edit.