A Google employee working for the Adsense team has written some useful tips in WMW related to the new Adsense for Referrals program:

1. If an advertiser ends their campaign or runs out of budget, those referral ads become unavailable and will stop showing on your pages.

If you’ve selected ‘Pick best performing ads’ or you’ve selected keywords or categories for your ad unit, then we will rotate different referral ads to fill the referral unit. If you haven’t checked that option, your page will display an empty space where the now-unavailable ad used to appear.

To maximize your opportunities to generate referral earnings, we therefore recommend you check the ‘Pick best performing ads’ box when generating your referral code.

2. However, if you select an ad unit size that is only available for Google product referrals and check ‘Pick best performing ads,’ there’s currently a system error that will cause no ad to be displayed.

Our [Google] engineers are working to fix this as quickly as possible, but in the meantime, if you’d like to refer users to these Google products, please uncheck the ‘best performing ads’ box when generating code for that referral ad unit.

Note that the referral products available to you may vary depending on your location or language - you can find a full list of the referral products available for you by visiting the AdSense Setup tab and choosing Referrals as the product.

In order to maximize conversion opportunities, choose the combination that best reflects the profile of your users. Please note that if you select ads for a location other than the one your users are in that you may not get paid for the conversion.