Exactly! If you have an open source (or any other!) portfolio, you're already ahead of 99% of the people I interview. Odds are that I won't actually look at your code. Instead I'll ask you technical questions during the interview about the project itself: architecture, design, problems you ran into and how you solved them. How you handle bug reports, etc.

Pretty much the same thing I do with other projects on a resume, but much more focused.