Good example of written Rails coding standards?

You might have a look at the Community Rails style guide. It's fairly extensive and up-to-date with current trends in Rails development.

Here's a style guide I rather like, Ruby-focused but definitely of the "Rails generation" of Rubyists:

Elements of Ruby Style

Credit for which belongs to Pathfinder Development:

_{(source: pathf.com)}

UPDATE. Two years later, I have found a better guide, brought to you by the copycopter project