This is actually a topic I suggested for this year's NBI, and a number of other NBI bloggers have posted recently about it:
Arcadian has a good write up on the topic of how these two can co-exist. Mainly if the PVP aspect isn't mandatory and doesn't block the PVE guys from any content.
Doctor Hannah doesn't mind "Responsible" PvP which is difficult given the douche-baggery that comes with the territory.
Doone has a piece on open PVP and fairness which has a great line: " Players will always find ways to avoid it if they really hate it, which might include not playing their game."
Looking for Playtime actually grew to like PvP, as the PvE element gets boring having done the same quests in different games.
My answer, after having experienced both consensual and non-consensual PvP (oh so dirty), is never.
Having open PvP all the time in PvE zones like in the old days of Ultima Online or in Wizardry Online (open PvP with chance of permadeath) really added to one's adrenaline as random encounters against Player Killers (PKs) were just that, very random. Unlike in say, a First Person Shooter deathmatch where you -know- you will be fighting other people, in MMOs it's a matter of some decision making and a lot of luck. Some days you might not run into anyone, let alone PKs. Other days you may have the misfortune of being hunted by a whole slew of them - usually for no reason other than "they were bored".
Doone's piece on fairness also rings true here: often a newbie in a FPS is still capable of serious trouble against a veteran, and usually the teams auto balance each side to keep the numbers even. In MMO's a low level and low geared character has almost no chance against multiple max levelled and epic tiered opponents. Not all PvPers are douchebags though (and not all PvEers are not). I've had highly wanted criminal characters and soldiers of an enemy faction be very civil and helpful to me (or at the very least ignored me seeing my low level) but they were few and far between the more common, PK thug type.
I agree with Arcadian that when developers mix PvE and PvP it's because they are lazy and hoping to get free content by feeding PvEers to PvPers who serve as a luck-gate to whatever PvE objective they were after. However this doesn't change the fact that it is still up to the individual player on how to behave. Do you really leave your morals and decision making when you log into your MMORPG? Do you get joy from ruining someone else's day? If you grief someone enough in game (because you were bored or just because the game wanted you to kill x people this week) that they commit suicide in real life, do you think that's on you? Of course it is. That there are people who think otherwise is part of the reason why these two game styles should never be found together.
Let the killers go kill each other in their own little room(s) while everyone else enjoys the actual game. Otherwise, just advertise your whole game as PvP focused so the rest of us know not to bother playing it.