For the longest time, I've used left-aligned labels in my forms, like this:
I did this because I thought having the labels left aligned made it easier for readers to scan the list of labels.
I've done a bunch of research and found a lot of discussion, including this UX question, which compares and contrasts the approaches, but nothing definitive.
In particular, I haven't been able to find anything to indicate why Billy Hollis changed his opinion from left- to right- alignment for labels (apparently he switched around 2007).
I figure Billy's too smart to have made the change without evidence - so I'm interested in seeing that evidence for myself.