I just changed info-level logging to warn level in my production log and it was quite enlightening! I found there were a lot of weirdly formed requests and in particular a lot of requests like this:
ActionController::RoutingError (No route matches "/undefined/ firebug.html" with :subdomain=>"www", :method=>:get, :domain=>"mydomain.com", :host=>"www.mydomain.com"}):
(domain name changed to protect the innocent)
Anyone seen this or something similar?