For GNU emacs >=23.1 url-retrieve is called with the wrong arguments

In both emacs 23.1 (Ubuntu 10.4) and 23.4 (debian testing)
url-retrieve is defined like this:
 (defun url-retrieve (url callback &optional cbargs)

In xml-rpc.el the url-retrieve method is called like this:
  ...
  (cond ((boundp 'url-be-asynchronous) ; Sniff for w3 lib capability
         (if async-callback-function
             (setq url-be-asynchronous t
                   url-current-callback-data (list
                                              async-callback-function
                                              (current-buffer))
                   url-current-callback-func
                   'xml-rpc-request-callback-handler)
           (setq url-be-asynchronous nil))
         (url-retrieve server-url t)

Ie. if url-be-asynchronous is defined, url-retrieve is called with t as
the second argument, and this will fail because url-retrieve-internal
tests the argument like so:
  ...
  (if (not (functionp callback))
      (error "Must provide a callback function to url-retrieve"))