Just published update to our windows phone game (Atoms 3D) on AppHub and its not displaying ads. It displays ads if I deploy the same XAP file directly to my developer phone. There's also a slightly newer version of the MS Advertising SDK 6.1.320 available for download from the version I downloaded a few weeks ago.
I've emailed pubcenter support to see if this release fixes this issue.
Another developer has reported the same issue on the pubcenter forum: http://community.microsoftadvertising.com/forums/p/72819/110806.aspx#110806