This is a bit tricky. I voted for all of the above, but there are a few sticking points even to that. I agree wholeheartedly with Josi's reasoning of potentially creating an outlet for OOC tension and frustrations to be directed towards killing someone's character. There's also the potential outlet of one player assuming such OOC to IC behavior, resulting in accusations being made, creating a mountain of drama from a mole hill of misunderstanding. OOC drama is never good.
However, as Ligeia said, even if they're informed of the possibility of character death beforehand, there's a good chance that a large majority of players aren't going to want they're characters to die, understandably so, and thus even in situations where death would be imminent, players will not agree
Moraline had a great idea, placing the characters in a GM appointed coma for a length of time and with physical/psychological damage to be determined by the GM based on whatever event they encountered to put them there, and then let them decide if they die. But there again, most people aren't going to agree to have their player killed.
So, the "With consent from both parties" is 50/50 for me. On the one hand, you might not get too many players whose characters may die at a moment's notice without their consent (or you might get very shallow, hastily thrown together characters as a result), but on the other hand, if you have to have consent, is anyone REALLY going to agree to allow their character to be killed off?