I have heard yin/yang a lot in aikido and it makes perfect sense to me to consider it during practise. As we all know, it is very difficult to deal with a weak attack. The stronger (yin) an attack is the easier it is to overcome it with gentleness (yang). Also, if you wish to initiate a response you need to be aggressive (yin), and then most people try to respond with an aggresive (yin) response, at which point you can then change to yang. Make sense?
Tai-chi Chuan is very definately ties in with yin and yang, and at a recent push-hands game I could really feel the yin and yang and the way that a persons force can be taken advantage of. Ueshiba is said to have the view of 'I am the universe' and that he had attained harmony with it and, like the universe, disharmony (e.g. through an attack) was dealt with by him in a natural manner through the tendancy of the universe to restore balance. That is why aikido as an ideal is undefeatable.