My buddy Curtis wrote a good article on Why Rails Feels like a Developers Only Playground.
Yeah the guys I work with at the Fv.rb are joking, but otherwise it seems to be a mentality that goes through dev's heads. They can't design but don't want to take the bit of extra time to help someone get up to speed. Most designers are more than smart enough, and many are willing, to learn some new technologies if someone is willing to give them a helping hand.
I don't think that devs aren't willing to help someone learn, in fact, I think that the Rails (and indeed, programmers in general) are more than happy to help out.
However developers know how developers think, and helping out someone who knows how to program (sorry, but HTML and CSS isn't "programming") is a lot different than helping a non-developer. Speaking as someone who tried to help their drama major ex-girlfriend (not major drama ex-girlfriend, that came later) through a simple while loop to increment a variable, I can attest to you 100% that different people's brains work differently, and that creative people and programmer-type people sometimes have a huge chasm of grokking between them.
Rails doesn't feel like a designers playground because it's a web programming framework, designed by developers for developers, and if you ignore some of the cool CSS and HTML frameworks built in and around it, there's really not much there for designers doing designer work in rails. Maybe I'm just daft, but other than pointing to the directories where various templates are stored, and showing someone what loops look like and what to style, I'm not sure that rails has anything over any other framework design wise. IE: none.
Now designing a site is a completely different matter. You want a good, knows their shit designer like Curtis involved in your project from the start, and maybe that's what the rails community is missing, someone to help shape their programmatic designs based on good web design. This involves site setup, layout, page headers and meta-data, etc, but, IMHO, doesn't infringe much into the "programming" world.