[Pitchwife]

Thanks for the comments, skip and Boro.

Well I think it's a great exaggeration to say that Frodo spends the novel becoming like Sauron, seduced by power.

My sentiments exactly. However, I think there's a grain of truth in this exaggeration, if you dig deep enough - which is the purpose of this thread.

skip: The influence of the ring, I feel, is rather like a force from the outside that Frodo is resisting stoutly, but in the very end gives in to.

Boro: But, I think the other question is, is it because the Ring is just that powerful, or does everyone's will have a bit of Sauron in it?

The point is, according to Donaldson's theory there's no clear dividing line between inside and outside in fantasy. If Sauron is a part of Frodo, so is the Ring. To quote Hermes Trismegisthos, what's outside is also inside, and vice versa. Maybe the Ring only is so powerful because everyone's will has a bit of Sauron in it.
Boro, I think Donaldson's statement indeed hinges on Frodo's critical failure at Sammath Naur - the closest he ever got to becoming like Sauron (or the little Dark Lord in his soul finally getting the better of him). Sadly, we're never told what went on in his mind at the moment, what visions and promises the Ring evoked to seduce him. Sam saw himself turning Gorgoroth into a garden - what would Frodo have done, if he could have claimed the Ring and got away with it? We'll never know (but feel free to speculate).