In this thread on Classic-Castle we've been speculating a bit on this. Athos noted that the announcement poster looked "more like Lord of the Bracelets." There are various possibilities for existing LEGO elements that could stand in for the ring, like a gold 1x1 round plate, the new version with a hole, the new 1x1 round tile (perhaps printed), or the Clickets ring. These could all be clipped onto a minifig hand, or even (somewhat) grasped.
As noted above, though, those are huge. On the other hand, a truly fig-scaled Ring would be unmanageably small. Of course, the Ring spends most of the Lord of the Rings on a chain around Frodo's neck, so perhaps it might simply be a printed detail on Frodo's torso, like these examples of the amulet worn by the KK2 Guardian, or Yoda's little flute. There's also an existing 1x2 printed tile that would work perfectly as the Ring on a chain.
Anyway, that's all just speculation. What are your thoughts?
Edit- Some additional thoughts that came up in that discussion thread. There is a Harry Potter torso where Hermione is wearing the time turner. Also, Captain Jack's compass was suggested as a comparison of a small important object for minifigs to handle, and there LEGO used a printed 1x1 tile. Two-face's coin is another great comparison.