I would suggest to make SimpleXMLRPCServer.SimpleXMLRPCServer.register_function a decorator function.
See the attached file for the solution I wrote (l.209-240), which also works with the current syntax:
@server.register_function
@server.register_function('name')
@server.register_function(name='name')
or:
server.register_function(func)
server.register_function(func, name='name')
server.register_function(function=func, name='name')

So as far as I've tested it (py2.6), it is fully backwards compatible and supports decorators in addition.