Silver makes the case that the polls at this point don't necessarily mean much, and you can get big swings in voter preferences in relatively short periods of time. And that's true. What I think is different is Republicans are tuned in to a much greater degree than they were at this point in previous nomination contests. You can see that in polling when you ask whether voters are paying attention, and you can see that in ratings for the debates. The idea that voters aren't tuned in yet and won't make up their minds till January or later may not prove as true as it has in the past.
Because of the higher level of interest and attention this year, these early polls may be more predictive of what's likely to happen.
The second point is Trump isn't only leading in national polling. He's leading in every state poll I've seen. He seems to be ahead in Iowa, in New Hampshire, in South Carolina, Nevada.