My current company has a "jeans and nice shirt" casual dresscode, but as an interviewee, I would expect someone to wear a nice set of pants or slacks, women - a blouse or very nice shirt or a dress/skirt, and men, a dress shirt and tie.
Once they get the job, they can wear a polo or nice shirt and jeans like the rest of the us.
I was in a position to conduct interviews for about 3 out of my 6 years here. I'm kind of glad I don't have to do that in my current position anymore....it was fun sometimes, but always a little uncomfortable knowing my decision about someone could be the difference between them having a good job, or not being able to pay their bills.