Our terms and conditions contained a clause that prohibited apps participating in Ad Exchange to use their credits to promote anything else than themselves. After some consideration we decided that this clause is too strict for no particular reason. We’ve rewritten that clause and now it looks like this:
4.1. Apps participating in the Ad Exchange Program can only have ads advertising official Windows Phone 7 Marketplace apps. Advertising of other apps or services is prohibited.
This means that now you can leverage popularity of one or several of your apps to promote your newer or less popular apps. A great way to boost exposure of that great app that went under the radar for some reason.