Well, since adding new keywords or operators is very sensitive, and the existing ones are rather exhausted, it won't be handy to propose a new syntax...

One last idea I might have : what about something like

    def myfunc(a, b, c = yield []):

I'm not expert in english, but I'd say the following "equivalents" of yield (dixit WordWeb) are in a rather good semantic area :
*Be the cause or source of
*Give or supply
*Cause to happen or be responsible for
*Bring in

Of course the behaviour of this yield is not so close from the one we know, but there is no interpretation conflict for the parser, and we might quickly get used to it :
* yield in default argument => reevaluate the expression each time
* yield in function body => return value and prepare to receive one

How do you people feel about this ?

PS : I've heard some month ago someone who compared the new high level languages as new religions - with the appearance of notions like "py-evangelistes" and stuffs - whereas it had (in his opinion) never been so for older languages :p
That's imo somehow true (and that's rather a good sign for those languages) ; I feel phrases like "pythonic" or perl's "TIMTOWTDI" have gained some kind of sacred aura ^^