goodroi - 4:35 pm on Jan 4, 2013 (gmt 0)
I doubt underscores will ever be preferred over hyphens due to the usability issue. Hotlinks are underlined by default and that can make an underscore not as visible to users.
From an SEO ranking point of view, last year Matt Cutts did say that Google handles underscores and hypens differently so it could have a slight impact. He was clear that the impact would be minimal and there are bigger issues for most webmasters to work on.