I almost always ignore players who are fighting pve mobs when I'm looking for some player killing(unless it's someone who killed me while I was doing PvE :p). I also only attack players who are at almost full health/mana(unless ofcours they attacked me first) and when someone politly asks to stop, I will. At least I stick to these rules when I'm playing on PvP servers cause there are no other ones avaible(I have not yet encountered any PvE private WoW servers)

[Linked Image from miniprofile.xfire.com]