How much does nipple choice affect wheel weight/stiffness? Would I be a fool to replace the nipples on a Dura Ace 7900 C24 wheelset with a set of DT Swiss anodised alloy nipples, in the interest of aesthetics? I want the bike to look nice, but don't want to compromise on performance!
Alloy would save a tiny amount of weight. I wouldn't expect it to make any difference to wheel stiffness, so long as you lubricate the threads when building it (alloy nipples are stiffer to turn otherwise, so it's harder to get the tension right). Brass is stronger, but you shouldn't have to rely on that to get a stiff wheel.

But long term, alloy is more likely to seize.

http://sheldonbrown.com/wheelbuild.html#nipples

http://singletrackworld.com/forum/topic/alloy-spoke-nipples-or-brass