I'm not sure I agree with that. Two follow up points:
1. I think an online portfolio is a great idea: I've hired a number of designers, too, and their portfolio is the first thing I've looked for.
The difference is a designer portfolio can include their professional work, whereas a programmer is limited to their open source contributions. I wouldn't judge a designer on their personal projects: "Hey, designer, I want to see your work, but only the stuff you've done on weekends and in the evening." It doesn't do them justice, and I'm basing my hiring decision on a subset of the work. In the same way, requiring open source contributions to gain entrance to the interview process is limiting.
2. If the programming culture required an online portfolio, that is essentially requiring programmers to have a second, unpaid, part-time job. I do not think you can generate enough great code in a few days to get noticed.
I do agree that some sort of portfolio would be handy: I just don't think an open source only portfolio is the way to do it.
