But there is absolutely no reason to think that simply by the act of signing this pledge, Trump will somehow be legally bound to not run as anything but a Republican in 2016. He won't be.
This pledge is not, as my colleague Bob Costa notes, a legally binding document. It's like the sort of pledge you get your kids to sign that they will do their homework, make their beds and eat their vegetables before they can play with your iPhone. It's a statement of intention, but not a binding one.
Do I think Trump is secretly plotting a third party bid if he winds up not winning the Republican nomination? No, not at present. Do I think that Trump believes that he will be bound by this pledge from running if he decides that's what he wants to do? Absolutely not.
What in Trump's relatively short time as a presidential candidate (or much longer time as a person who takes positions on various issues) suggests that he would feel at all compelled to abide by a pledge put together by the head of a party that he is a) only a relatively recent convert to and b) can't really hurt him, financially or otherwise, if he decides to break it?
Trump's entire candidacy is premised on how he doesn't owe anyone anything and how no one can tell him what to do. So, for the moment, it's in Trump's interest to play nice with the GOP establishment -- since it might allay some fears of voters who are thinking about being for him but wonder if he is, actually, one of them.
But if Trump, at any point over the next few months, feels hard done by the GOP establishment -- or if his poll numbers begin to fade -- is there anything in what he will sign today that keeps him from breaking the pledge not to run? No.
Yes, Trump would have to answer for his broken pledge among some Republican voters who took him at his word. But given Trump's anti-establishment message, there would almost certainly be a big chunk of disaffected Republicans and Republican-leaning independents who would likely take his side in a fight with the national party.
Point is: This is a good day for the RNC, which is showing its ability to bring some order to what has been a wild race to date. But don't assume that Donald Trump signing a piece of paper in front of a bunch of cameras is anything more than a nicely-turned bit of political theater. Because, well, it isn't.