No referral program would interest me. In general, referral programs make me feel icky, as a referrer. Probably because when I’m on the receiving end, it instantly devalues someone else’s recommendation when I discover they get something out of having pitched a service to me.
I think that last point is worth deeper consideration in Beeminder’s specific case: Beeminder is entwined with money by design. People already have a hard time understanding the appeal of paying money when you fail to achieve your goals. So when a trufan is waxing lyrical about the wonders of Beeminder, I think it doubly undercuts their believability if they’re recouping some of their losses in the process. I’m not sure a referral program would actually increase new user numbers—it might even decrease them.
(Obviously that depends on how many more people would hear about Beeminder, how many would be put off by the friend getting a “kickback,” etc., which are probably really hard to quantify up front. Just a point I thought worth mentioning while you’re brainstorming.)