I also wonder, because I get about 1000 PSAs daily, and it would be good to try to replace them with new referrals.
Yes, just use the alternate ad link in the javascript. When creating the code through AdSense's dashboard, you set an alternate ad. I personally am considering just setting a background-image for the div my ads go in, this way if ads are blocked by the user, the div still shows up and a banner/ad is also displayed.
Or you could use <NOSCRIPT> <SCRIPT> Here the Adsense Script </SCRIPT> <NOSCRIPT> Here your alternative ad code </NOSCRIPT>
If the client machine has javascript disable, Adsense ads will not work anyway. With <noscript> </noscript> you can serve an alternative banner with <a href=... and <img src=... or any other kind of banner. But obviously, if javascript is disable, you have to forget about Adsense ads.
It is not working for me.. I think I'm doing everything correctly, if I change the google script for an image, it works fine, but when I put the script back, the space where the banner is supposed to appear is blank.