Apple has just launched the .Mac affiliate program: if someone signs up for .Mac by following a link on your site, you receive $15. It's that simple.
Affiliate programs have been powerful tools for drawing in new customers for subscription and e-commerce retail stores because the goal in those cases is lifetime customer value. It makes sense to pay relatively large commissions to referrers who produce single-year subscribers, who, in turn, are more likely to become multi-year subscribers. In other words, paying $15 to gain $200, $300, or $400 in eventual revenue doesn't seem silly.
Apple has certainly done the testing and run the numbers on this program. A $15 bounty for a new .Mac subscriber means there's a large universe of potential .Mac customers that Apple can't reach through its other advertising techniques; they must also be seeing a fairly high non-cancellation rate among referred .Mac subscribers.
Affiliate programs also have the benefit of stamping the imprimatur of the site that refers (the affiliate) to the site that pays (the advertiser). If a Mac Web site started showing a .Mac affiliate banner, it says to me that the site thinks referring people to .Mac and having them sign up makes sense for its readers.