render :text sometimes rendering HTML?

This one really stumps me:

def hello
  render :text => 'hi there'
end

Renders with content-type 'html/text' most of the time. But not
always, sometimes it has the type 'text/plain'.

For example:
http::localhost:3000/controller/hello/rob.txt # renders as text
http::localhost:3000/controller/hello/robots.txt # renders as html

the relevant line in my routes.rb is:
  map.connect ':controller/:action/:id.:format'

any ideas?? DId I hit a bug?