Wow, I just joined this forum, and find the exact same argument I was having with my friends about this class last session.
I don't think the Favored Soul knowing Knowledge(Arcana) is a typo. I figure they had to give them ONE of the Knowledges as a class skill since every class seems to have one, and on the surface it appears that Knowledge (Religion) would be the best choice for them, being holy charaters and all... but Knowledge(Religion) implies a character having precisely the type of clerical study and training in his/her background that the class eschews, does not need, and is held at arm's length by "proper" clerics due to not requiring in order to cast divine spells.
Of course, my DM disagreed with me and changed the skill to Knowledge(Religion), so I doubt my interpretation of the class is universal.