I believe the Product Owner is an important member of the team and needs to be as accessible as any other.
I've recently been confronted by the idea that because the PO is part of 'the business' (i.e.: the client) they should not be considered as a member of The Team.
Put another way: They should attend important meetings and most stand-ups but be limited in their involvement to avoid trying to control the project (as has become bad practice for BAs or project management.)
Should they be kept at arm's length or fully indoctrinated into the agile process?