I think I remember Tolkien did actually mention that possibility in a letter. I think he may have said that if Sam hadn't interrupted and if Smeagol had won the little argument with Gollum while Frodo slept, that the focus of the book would have then had to shift to Smeagol.
I always thought that Sam opened his mouth a little too soon during that episode.
-Kevin
He said something like if Smeagol had been given more time to win that argument and had found redemption, in the end he would have taken the ring from Frodo and then would have thrown himself along with the ring into the cracks of Doom to save his master.