This class may finally codify the line between implement and weapon keywords. We know that you don't get proficiency bonuses to implement attacks. But, the hexblade introduces a new thing in that it has powers that include both the weapon and the implement keywords.
I think we'll probably get a ruling at some point that basically says the following:
When using a weapon as an implement, it does not count as a weapon.
For example, when using Soul Eater, the hexblade would use a rod (or wand) to satisfy the Implement keyword, while using the Blade of Annihilation to satisfy the Weapon keyword.
But, if the hexblade has AIP (heavy blades), then he'd have, say, a scimitar as an implement, and the blade of annihilation as a weapon. The blade is being used as a weapon and not an implement, so it does not count for dual implement spellcasting. The scimitar is being used as an implement and so does not count as a weapon for two weapon fighting or defense.