It's Okay to Say "I Don't Know"

I've only been working in the tech industry for a couple years now and I'm starting to notice that people (and companies) have a very difficult time saying "I don't know" (or "no", "we can't", etc).  Why is this?  There's nothing wrong with not knowing something, as long as you have the skills and abilities to figure it out.

I was at a local BarCamp meetup a little while back and one the people there was talking about finding jobs.  At one point he said to never say you can't do something in a job interview or else you wont get the job.  I was baffled.  If you can't do something, why would you say you can?  At some point the truth is going to come out anyways.  Is this really what it's come down to?  A better question, do managers expect this when doing interviews?  Do they expect people to say they can do anything even though they can't?  I really don't know, but this kind of scares me.

I have a couple friends from Europe and they're just as confused by this behavior as I am.  Apparently they don't have a problem with this, if you have something on your resume you bloody well know how to do it, if you can't do something they expect you to say so.

And really, what is the problem with hiring someone that might not know every detail of everything the position requires?  If I were hiring someone I'd mainly be interested in their general problem solving and critical thinking skills.  Sure, you would need to have the basic skills for the job, but as long as it's obvious that you're smart and interesting in learning what you don't know what's the difference?  I'll take that over someone who doesn't care about the job and is just there for a paycheck any day.

What have your experiences been, have you noticed the same things?  Do you say "I don't know" enough?  Leave it in the comments.

"We need to accept our ignorance and say 'I don't know' more often" - Malcolm Gladwell, Blink