It appears that syntax coloring works on Posterous using syntaxhighlighter.

Doesn't quite work with emailed posts from Apple Mail, which inserts a lot of needless <div>s.

UPDATE: I figured out that you can go into the raw HTML editor and replace those <div>s with <br/>s and all should be well. But note that if you type code directly into the rich text editor, it will be wrapped in <p>s, which also must be turned into <br/>s—otherwise newlines will be inserted all over the place.

For kicks, a working sample:

class FakeFace
  def whatever(opts = {})
    opts[:key] || "nada"
  end
end