Thankfully it's trivially simple to do in Ruby. For this blog I really wanted a solution that lets me keep the text clean in the database, but marks up the posts when rendering, so after some searches I ended up doing this:

gem install hpricot
gem install syntax

Then I added this:

require 'hpricot'               
require 'syntax/convertors/html'
def filter_content(content)
    h = Hpricot(content)
    c = Syntax::Convertors::HTML.for_syntax "ruby"
    h.search('//pre[@class="ruby"]') do |e|
        e.inner_html = c.convert(e.inner_text,false)
    end
    h.to_s
end

Then all I need to do is call filter_content() with the post content, and put any code in pre tags with class="ruby".

(Hpricot is a great tool btw - I use it all over the place for mangling HTML.)

The remaining piece is just some CSS. The CSS was adapted from CSS found here, but I ended up changing most of the colors to match my editor settings better:

pre { background: #111122; padding: 10px; color: #228822; }
.ruby .normal { color: #fff; }
.ruby .comment { color: #005; font-style: italic; }
.ruby .keyword { color: #A44; font-weight: bold; }
.ruby .method { color: #44f; }
.ruby .class { color: #0c4; }
.ruby .module { color: #050; }
.ruby .punct { color: #668; font-weight: bold; }
.ruby .symbol { color: #ff0; }
.ruby .string { color: #4f4; }
.ruby .char { color: #F07; }
.ruby .ident { color: #fff; }
.ruby .constant { color: #0c4; }
.ruby .regex { color: #B66; background: #FEF; }
.ruby .number { color: #F99; }
.ruby .attribute { color: #fc4; }
.ruby .global { color: #7FB; }
.ruby .expr { color: #227; }
.ruby .escape { color: #277; }