I had suspected for decades that the Arkenstone was either Maedhros' Silmaril (in-world), or some sort of proto-Silmaril object which Tolkien later denied because it didn't fit in his mythology as he went on to build it. There are a few elements in The Hobbit that are a bit different or lots less scary/important/high-falutin than they are in LotR or The Silmarillion, let alone stacks and stacks of posthumously published fragments. There's the ring, which Bilbo just uses casually and that serves as an important plot device only insofar as it makes him invisible; the drunken Wood-Elves who are totally a hoot and nothing near as high and mighty and airy-fairily impressive as they are later; and the Arkenstone, which is sort of important because it's famous and a Glowy Stone of Glow, as you so nicely put it, but doesn't carry all the enormous weight of a Silmaril. Also, if it was a Silmaril, Thrain would sooner or later have had a polite visit from Galadriel (introduced by Thranduil, most likely), who would have made polite conversation, eyed the thing where it was on display, and would then have hurried back to the rest of the White Council, going 'People, we have a situation!', only more high-flowing and elvish. Actually, he would have had that visit anyway, even if it then turned out not to be a Silmaril, because the news of 'Glowy Stone of Glow found in the very depths of a dead volcano' positively shouts 'Simaril!!' to anybody who was around in the First Age, or indeed had done their history homework. Because there were three, and we know where they ended up: as the evening star, in the sea, and in a volcano. Maedhros' body would have burned away immediately, but a Silmaril of Feanor, containing the light of the Two Trees, would probably have survived, so whoever found the Arkenstone never saw a trace of dead elf.

I'm really looking forward to seeing, in the next two movies, how Peter Jackson handles the Silmaril question, and how the Arkenstone will behave -- because like the One Ring, a Silmaril is an object with something of a magical will of its own. If it does behave in any way (just as it was smoking slightly in Thror's hand), that's a point in favour of it being a Silmaril.

Also, if it is meant to be a Silmaril, Peter Jackson has some sort of anchor point to spin off Silmarillion movies from.