Thanks to everyone who voted (all 111 of you) and those perceptive enough to notice me giving up and putting a poll up was also an act of surrender.
The results were:
Always, instead of dying
Sometimes, instead of dying
Always, instead of fleeing
Sometimes, instead of fleeing
Always, if unable to flee
Sometimes, if unable to flee
Sometimes, if badly hurt and the player heals them
If the player calls for surrender
I'll be putting in the most popular result as a feature before the release of Unangband 0.6.4. Monsters will sometimes surrender if they are unable to flee: this will depend on a saving throw based on your Charisma, and whether you have the birth_evil option enabled or not.
Two other options: sometimes, if badly hurt and the player heals them, and if the player calls for surrender, are available through the existing 'O'ffer code. I'll add the ability to offer nothing, and have badly injured monsters respond in some way as well.
I may allow the option to hold back on delivering a killing blow, by attacking a monster to disarm. Hmm...
The next poll is a chance to tell me what features of 0.6.4 you like the most so I know where to polish things up for the final release. If you haven't had a chance to play, feel free to tell me what sounds interesting.