One thing I'd expand on here.

Don't just say "I don't know" and then stand there staring at me. It's amazing how often this happens, and I don't know if it's because people read this advice or what.

The proper way to do this is "I'm not positive about this but here's how I'd approach it". Then start working through what you can and be very verbal about what you're doing. Tell me exactly what you're thinking and why you're doing things.

The goal of an interview, at least at a good company, should be focused on seeing how you think, not what you have memorized. If you just say "I don't know" and nothing else then all I can grade you on is that you didn't know anything. If you explain your thought process, not only does that give me more to grade you on, but it also allows me to guide you. There have been lots of times where a candidate has been stuck but as he walked me through what he was thinking I was able to give him a little nudge toward the right answer. I consider that to be a good sign in a candidate.