Well I'll say this:
Spoilers Below You are warned!!!
Anyways, when Frodo and Sam reach the cracks of doom, Frodo claims the ring for himself and puts it on, but Gollum bites Frodo's finger off taking the ring, but ends up losing balance as he jumps around and falls into the fires within mt doom.
Now Im not sure if they will show this exactly the same way in the movie, but no one can be sure until there is official confirmation or when we see the film finally.
