Perhahs a few words coming from experience would be helpful:
Some of the answers that you will find are limited to the lengths that one, as an individual, is wiling to go when executing a technique or wrestling with another human being. It should always be considered that causing death is an option, if not for you, than for them. And the games that people play to replicate this can fall short of the reality of a moment where a life is about to be gained or lost. No one is there to say "yamae" and no tap out will do.
Quite true. Of course if one is incapable of achieving desired result while playing, then the likelihood of doing so when life is on the line is often more remote. It comes down to the individual. Getting back to my last post - if you can't "catch" the knife hand of someone who is not trying to kill you then the likelihood of catching it when someone is trying to kill you is a lot lower imho. Unless of course you plan on catching it with some other part of the body than the hand.
Brian: My experiences have been quite similar to yours. Ma ai and tai sabaki are quite critical elements when getting hit even once can be the fight ender (they have saved my life at least once). This to me is important when we look at how Aikido operates as a method of dealing with serious, focused physical conflict. Good to have another p.o.v. from someone who has actually used it.
Boon: You're biased, you were once a Shodothug.