I remember when I would ask for $50/hour for design work, people would without hesitation go for it, some startups even asked why I charge so low....eh, I didn't know better. While I don't have much experience with rails consulting, from what I've heard, people who can do both rails and ui design can EASILY go for $100/hour. Then again, I have a friend who just landed an ~$80/hour work doing some simple joomla/php work. To be honest, it sorta made more sense to me(even though I'm both a designer and rubyist) that designers made more. My belief is that it's more common for designers to be much more unique than a rubyist.
What do I mean? My theory is that it's easier to become another kick-ass designer who has a lot of people digging his work than it is to become a "popular" rubyist(I'd consider the guy who created will_paginate to be popular in the ruby world).
Am I the only one thinking in this way? ... it might sound a little ridiculous to some people. :)