Correct me if I'm wrong, but pre-commitment isn't an option in Newcomb's problem, so the best, the most rational, the winning decision is two-boxing.

By construction, Omega's predictions are known to be essentially infallible. Given that, whatever you choose, you can safely assume Omega will have correctly predicted that choice. To what extent, then, is pre-commitment distinguishable from deciding on the spot?

In a sense there is an implicit pre-commitment in the structure of the problem; while you have not pre-committed to a choice on this specific problem, you are essentially pre-committed to a decision-making algorithm.

Eliezer's argument, if I understand it, is that any decision-making algorithm that results in two-boxing is by definition irrational due to giving a predictably bad outcome.