So as long as Omega can maintain a greater than 50% accuracy, you should expect to earn more money by one-boxing. Since the solution seems so simple, and since I'm a total novice at decision theory, it's possible I'm missing something here, so please let me know.

Your caclulation is fine. What you're missing is that Omega has a record of 70% accuracy because Omega always predicts that a person will one-box and 70% of people are one-boxers. So Omega always puts the million dollars in Box B, and I will always get $1,001,000$ if I'm one of the 30% of people who two-box.

At least, that is a possibility, which your calculation doesn't take into account. I need evidence of a correlation between Omega's predictions and the participants' actual behaviour, not just evidence of correct predictions. My prior probability distribution for how often people one-box isn't even concentrated very tightly around 70% (which is just a number that I remember reading once as the result of one survey), so anything short of a long run of predictions with very high proportion of correct ones will make me suspect that Omega is pulling a trick like this.

So the problem is much cleaner as Eliezer states it, with a perfect record. (But if even that record is short, I won't buy it.)