An exciting thing happened today while I was stuck on a plane away from the Internet, which is why I'm so late talking about it — Google opened up its referral program to all AdSense publishers! (The referral program had previously been by invitation only.) You can read the official AdSense referral FAQ for all the details, and of course this is a hot topic of discussion in all the webmaster forums. Rather than just repeat what everyone else is saying, let me focus on a few different things.
Unlike most of Google's additions to the program, this one limits parts of its scope to US publishers only. All publishers throughout the world can refer new publishers to Google, but only US-based publishers can make money by referring the new Firefox and Google Toolbar button. As a Canadian and a Firefox user I can only hope they'll expand that part of the program to the rest of the world soon.
Also unlike ad units and link units, right now the referral buttons that you get from Google cannot be customized to blend in with your site. What you see on the referral page is all you can get in terms of colors. Hopefully this will also change.
The actual referral buttons that you place on your web page (you can have one button per page for each separate product that Google offers referrals for) are generated using almost exactly the same JavaScript code sequence as the ad and link units. First there's a script block that sets some “google” variables, such as “google_ad_format”, “google_ad_width”, and so on. These look familiar except for the “google_cpa_choice” variable, which is new. This script block is then followed by a reference to the same external script used to generate the other ads. Presumably when Google introduces color customization of the ads new variable will be set in the first script block as with the other ad types. Remember, you can't change the code that Google generates for you, though I'm sure a few adventurous types will try some different combinations just to see what happens.
Unlike most referral programs, you don't get a percentage of the money made by the new AdSense publishers you refer, just a flat fee whenever a referree hits the $100 mark in earnings for the first time. Chances are you won't see any money from referrals for a few months, just because of the time it takes for new publishers to get themselves setup with AdSense, especially if they're building new sites. Since you can't customize the buttons very much, you'll have to decide if it's worth placing the buttons on your page.
If you don't want to give up the space, consider using them as alternate ads instead, so that the space is not wasted by public service announcements or blank areas. It's easy enough to build the HTML file that does nothing but show one of these referral buttons, and several of the formats exactly match existing ad unit formats. That might be the simplest way to take advantage of these ads for some of these.
Now, if only I could get all my the readers of my book to use a referral button when they sign up for AdSense…
Eric Giguere is the author of Make Easy Money with Google, a real (printed!) introductory AdSense book for non-technical people, available at all fine bookstores. Be sure to download the free sample chapter for more information about the book.