Here's what I see for the Shaman: the basic shtick seems to be that the shaman's powers attack one targeted enemy, and then cause the shaman's spirit companion to give off some sort of effect. For example, the shaman might cast a ranged spell on one target, and then until the shaman's next turn all enemies adjacent to the spirit companion grant combat advantage. There is no restriction that requires that the enemies attacked be the same as the enemies incommoded by the spirit companion. This lets the shaman sort of be in two places at once- he can cast a spell over here, and inflict an effect over there, all in the same action. The shaman can also command the spirit companion to make attacks as well, but at least for this shaman most of the powers that do that are at wills.
Its possible that the spirit companion previews some of the rules for summoning. Technically its not a summoned creature, its a conjuration, but the fact that it uses a single hit damage threshold instead of a set amount of hit points is one quick way to simplify damage for extra creatures created by spells.
I guess we'll know all the details on Monday.