I once had an NPC stay on the ground after one of those bar brawls. They surrendered, gave me my winnings, and then followed me around (at my "Follow me" command) never getting up off the floor.
That was the closest I have ever gotten to an NPC really yielding. It was obviously an unintentional minor bug, but it was a good one. Made it seem real.

Believe it or not, NPC enemies sometimes DO have the two brain-cells required to realize that taking an opportunity to run is in their best interest. So far I've only had a random Orc I met on the road stand down (only to be killed by nearby Ice Wraiths) and a Blackblood Marauder who ran off in fear.

Wait, you're saying you can't trust a desperate bandit? OMG, who would've thought it?

Anyway, I've had yielding bandits run away and hide a few times now. They won't attack unless you go looking for them in whatever corner they're cowering in. Usually happens when they're the last one left. Thieves have yielded and walked away too, saying "I'll let you live. This time."

This guy here actually plays as a good character. Level 9 and hasn't killed a single thing. http://www.youtube.com/watch?v=y2d2KRIUYCM