when I first read your post, I had tons of things flooding back to me, but everyone here has covered it! MY mistake at my last interview (but I have been there 5 years so...) I bought a "power suit" at Sears for the interview. I forgot to see what would happen when I sat down. What happened was that my slip was too long for the dress! So I tried to roll the waist slip up around my waist but it was silky material and I panicked that it would slowly unroll. So I pullled it off in the car. But through the whole interview I was very self-conscious about how short the dress was when I sat! (I kept the suit jacket on to maintain some dignity and luckily- I never where "nude" colored stockings to something like an interview- had on black or something dark I'm sure!) Got the job and who knows what the too short dress did...
But DON'T YOU DO IT! I'TS AN ABSOLUTE NO-NO!! No showing extra skin! Definately wear a skirt or dress even if the job doesn't call for it- you'll stand out for the effort! but make sure it is no higher that a few inches over your knees!