I give a short and fairly simple whiteboard programming problem when I interview programmers, and I feel like the results have correlated really well with the programmers' eventual success at our company. Of course, there could be false negatives.

I watch them work and help them out as they go; I'd get a ton less info if I just looked at the finished product.

We do also have a couple of "off line" coding tasks, which are even more informative (if I had to ditch one, I'd ditch the live one).