I've blogged about surprise rewards and out-of-context user delights. On a related note, Sam Decker just blogged about "unexpected touch":
Now imagine if after a month after using iTunes, Apple knew which artists I liked and sent me a coupon for an upcoming local concert for one of my favorite artists. That goes outside what I ever expected when buying an MP3 player. That would be unexpected.Doing the unexpected makes people talk about you and usually doesn't cost a lot.