There is a small thread at WebmasterWorld with two advertisers complaining that their US based ads are not running as often as their UK based Microsoft adCenter campaigns. In short, the US campaigns they were running, in comparison to the UK campaigns, were getting a tiny fraction of impressions and clicks.

One advertiser emailed Microsoft and received this interesting response:

We know adCenter has hit campaigns for low quality ranking factors in the past, but to have two campaigns, exactly the same, with the difference being UK versus US, that seems interesting. What I find even more interesting is that Microsoft here is suggesting the advertiser build a brand new campaign and just copy over the data.

Forum discussion at WebmasterWorld.