Right, been looking at this and eventually found the problem. When I originally implemented the pubcenter control there were know issues with the both versions so I took measures to work around them, it's been updated once since the initial push and
i've updated it again for the next point release.
In the version available at the moment it looks like they have fixed one of the issues however in such a way that it now causes an exception if you try to initialise the control twice. Simple to fix.
So you can either download the source and patch the code yourself or wait for the next point release which should be fairly soon (obviously depends on your urgency).
To resolve this edit "AdRotatorXNA.cs" and in the section that creates the pubcenter control change the following from:
if(!AdGameComponent.Initialized) AdGameComponent.Initialize(this.Game, PubCenterAppId);
And the Pubcenter ad control will now continue serving ads until it either runs out or looses the network :D
Hope this helps.