Weak association with leukemia, which is why I wouldn't move close to one if kids are in the picture. There's just never going to be a great way to collect data to prove causation. You live next to it for 8 years and 300 yards away, the next subject lives 500 yards away for 3 years, and the next 350 for 35 years. You'd have to take a cohort of people and split them up over 35 years and put 10,000 of them under a power line and check for differences. It just isn't going to happen.
It reminds me (because the mrs. has a bunch of physician friends) where it's something that a dr. would say that there's no proven link, but they wouldn't live close to it, either.