From what i read about PvP it seems there will be open guild warfare. And maybe some areas where nonguilded chars can have open PvP, too. All the rest PvP is based on challenges.
I would like to add another way for nonguilded chars to join open PvP. The contractors.
A contractor is either not guilded or in a secret guild. He can align himself with a guild given apporvement of the guild. He then is bound by the same rules that applies to that guild regarding PvP. In other words he counts as a member of guild for PvP. The difference to an actual guildmember is, that he cant access any guild privileges, like the guild chat, special merchant offerings, heraldic equipment(flags, etc) and guild housing.
There are some things where problems still need to be ironed out. Like how much delay needs there to be to prevent aligning-killing-leaving runs. How to mark them as aligned to which guild, if they are to be marked. Should there be a maximum limit of contractors for a given guild size/wealth?
Beside the PvP effect it will also allow to play out assassins and bodyguards a little easier since you dont need to found a guild. And secret guilds could work for both sides in a war without compromising themselves.