It can easily be the one from the book -- their first encounter in Emyn Muil.
That one would have to include Sam as well, though. (Maybe it does, and Matt's street just isn't completely reliable.) As a plus point, it wouldn't only recreate the scene from Book IV, it would also depict the famous Alan Lee illustration, as PJ took great care to reenact it in the movie.
Or it's the one in Sammath Naur, with Gollum biting Frodo's ringbearing finger off. That one is also from the book, so no complaints there either.
It could be the two of them struggling immediately afterwards. Not in the book, and not as iconic as the first two choices anyway, so hopefully that's not the one.
Finally, it could be their fight immediately after Shelob's lair, or on the slopes of Mt. Doom (again with Sam involved). Sort of canonical again...
Whichever it is, I'm glad we haven't gotten another Gandalf vs. Balrog before the plaque has managed to sell out; and I'm ecstatic we're not getting yet another Lurtz piece after all.
First Maul, now this -- I'm stoked, stoked!