Mako

Spank has it on the money here. My stock response for something I don't know the answer to is "Being honest with you, I'm unfamiliar with x, however I'm very intuitive and capable of learning on my feet. For instance..." and reel off an example of when you've used your initiative to learn something new or had to pick something up fast.

I find that interviewers react well to telling them about real world situations that you've been in before, hence the buzz around competency based interviews.