It may be a matter of semantics, but I tend to believe, or view aikido simply as a methodology for learning martial skills.
The things/skills you learn that become second nature or just that, skills...not "aikido". aikido is simply the way of study in which we learn these things. So from my perspective, aikido cannot become second nature and you can never "perform it" or do it.
It may be somewhat philosophical, but once things become second nature or become apart of you, they are simply that....YOU.
To me while it may seem like splitting hairs, i believe it is very important to really understand this if you ever hope to acheive synthesis or mastery.
Also, now as I think about it, I am not really comfortable with the words "aikido technique" and "random situation", as I believe that there (as I already stated), that there are no aikido techniques, and also no situations are truely random in nature (or suprising).
I think part of refining the art is that as you grow you can more readibly identify conflict or the potential for conflict and therefore, it really does become a more subtle art that requires small movements, subtle kamaes, gestures, or actions, not kotegaeshi's are Ki ai.
I think though, that Matt's question was much more simplistic in nature about "technique and situation". I fear that I have hi-jacked his thread and turned it into a "internal" type answer. (sorry for that )