There is a known problem with Bing Ads that the Z order will always be at the top. There is a fixed API that requires the use of a new attribute to resolve this.
<UI:AdControl Width="250" Height="125" ApplicationId="..." AdUnitId="..." UseStaticAnchor="True" />
Blog about fix
So you have to get the new SDK, and then the ad control needs to set the UseStaticAnchor flag to fix this issue.