Update 2: The issue has been resolved, according to our tests and tweets by @ZuneSupport. Kudos to Microsoft for a speedy resolution.
It is being reported widely on twitter that users of the desktop Zune software are unable to log in to their account. It seems that users are stuck in an endless loop of them accepting the terms of service, checking their communication preferences, and then the dialog pops back again.
When trying to log in, these three screens loop endlessly
Locally stored content can be played back, but streaming or downloading content using the Zunepass is currently not possible. Zunepass users can use the web interface for the immediate music fix in the mean time.