The official announcement:

Zune Marketplace will be unavailable for approximately 4 hours to upgrade the service starting:

Local Times of Maintenance

0200 Pacific Time 09/10/2007

0500 Eastern Time 09/10/2007

1800 Japan Time 09/10/2007

0900 GMT 09/10/2007

EDIT: As Jonnie has noticed in his comment below, the Xbox Live service will be offline during the exact same hours as the Zune Marketplace. As you can read in this Major Nelson 's article, the Xbox live maintenance won't bring anything new to the service. The article says it will effect the  "My Xbox" place on Xbox.com and set the forums to read-only mode. FYI this blog belongs to a member of the Xbox team.

So I suppose we shouldn't expect anything new either.