It's worth talking about in a wee bit more detail, but

@new_user.card_limit_for_free_trial = ab_test("user-card-limit-may-2012", [8, 15], :conversion => "purchase")

gives away a bit of it. (That's literally all the code. The old default was 15.)

Funnily enough, I mentioned I might do that test eventually on HN ( http://news.ycombinator.com/item?id=3864079) and at the time panned its likelihood of success. Shows what I know, for the 495th time with A/B testing.