"[A]rbitrators are likely to be less familiar with ERISA's nuances and an errant arbitration decision could raise a number of difficult compliance questions for the plan's fiduciaries.... [It] is not clear whether prior arbitration decisions would have any preclusive or precedential effect on the plan's fiduciaries or its participants ... [W]hile federal courts may bind federal agencies, ... an arbitration decision may not have the same binding effect on an agency auditing the plan."