A computer programmer with such strong skills and so much specific experience that they are the equivalent of a rock star in the domain of software. Many people play guitar pretty well, but only a few become rock stars. These programmers can develop more software than 5 - 10 newly hired regular programmers because they know what needs to be done and how to do it. They also might set the architecture of the product that dozens will build upon. Usually associated with dot com websites.
Jeff was the guy behind ebay.com He's a rockstar programmer.
A rockstar programmer is a programmer who becomes popular not because of technical achievement, but rather by the volume of fanboys who blindly consume his/her products. He or she poses in numerous techie magazines, giving interviews about nothing but Web 2.0 poetry, and attending totally useless "conventions".
DHH is a rockstar programmer.