Caching actions without layout can be complicated when multiple request formats are used. In particular, an HTML response may use a dynamic layout, in which case you want to use the :layout => false option. However, other formats (such as XML) don’t use a layout, but the :layout => false option to _caches_action_ does not properly cache the body in this case. To solve the problem, create two caches action statements:

caches_action :show,
              :if => lambda { |c| c.request.format == :html },
              :cache_path => lambda { |c| c.cache_key },
              :layout => false   caches_action :show,
              :unless => lambda{ |c| c.request.format == :html },
              :cache_path => lambda { |c| c.cache_key },
              :layout => true

Also, relying on the accept header may not cause the action caching module to detect the appropriate format. Try this in your application_controller:

before_filter :set_explicit_request_format
def set_explicit_request_format
  # Set format explicitly from accept header, unless it's already set
  request.format = :html if request.format == :any
  params[:format] ||= request.format.to_sym.to_s