chickonaroost, I had the same problem with one of my late cats; she would poop and pee right out front of the litter box without fail, no matter how clean I kept it.
She was a shy cat and didn't get along well with the others (they picked on her) so she didn't like the smell of the other cats, which I guess she could still pick up on even when the litterbox was clean.
I just put newspaper in front of the litterbox, and she would go on that. Not the most ideal situation, but it was the only thing that worked and did make it easier to clean up.