Very interesting idea. I would personally be VERY conservative about giving aspects to items, but it would certainly make sense in some cases. By giving an item as aspect, it goes from a mere item to some degree of an NPC. It gains a will of it's own and is no longer just an implement. For example: Lord of the Rings One Ring would absolutely most definitely have Aspects. Harry's Duster would not. Your Mjolnir could easily swing either way. I'd ask: is the hammer going to start making decisions, taking actions, and driving your story forward without input from the PCs? If so, then it should have Aspects, and probably even go though the whole NPC creation process.