I just caught this thread. This might be obvious, but an example of a yonic symbol would be anything rounded and open--the 'Omega' sign, an 'O', a opening to a tunnel, etc. It's intentionally vague, of course, just like a phallic symbol is interpreted as basically anything that's stick-like. Sometimes it's just rounded, not even open--some consider a dome shape to be yonic in the same way some see skyscrapers to be phallic. It might be interesting to note that a lot of both religious and capital buildings seem to mysteriously contain both (stick-atop-dome).
With symbolism like this my initial reflex is to roll my eyes, thinking about grade schoolers making immature, finger-in-hole gestures. But quite a lot of psychology has shown that these sorts of symbols (and a host of many others) do play a part in a sort of unconscious communication in the mind.