Eric brought this question up to me. I tend to think promise-breaking is lying. Here's the form a promise to someone takes: "I promise that I will do x." In effect, this is a guarantee that you will do x. And since 'I will do x' is equivalent to 'it is true I will do x', it follows 'I promise that I will do x' is equivalent to 'I promise that it is true I will do x'. To intentionally break a promise then is a form of lying; for you are guaranteeing something is true when you know it is not. In fact it may be an even worse form of lying, since you are not only saying intentionally what is false, but by promising it to the other person you are guaranteeing to someone that it is true. This is probably why people are even more disappointed when someone breaks a promise than when they tell a lie without a guarantee of its truth.
Of course, unintentionally, something may hinder you from fulfilling your promise. But then you are not morally culpable for failing to fulfill the promise. This is just a more specific case of telling someone something you believe to be true but, through no fault of your own, you don't know the details and it actually turns out to be false. When you promise something to someone you don't know that, while you are on your way to fulfill your promise, a large set of goons determined to stop people from fulfilling promises will pop up out of the corner. So it's not your fault for not being able to fulfill the promise.
These sorts of cases show that lying should be construed more subtly as intentionally telling a false-hood. And promise-breaking should be construed as intentionally failing to fulfill a promise. With this in mind it is plausible to think of promise-breaking as a more specific form of lying.