What's an obligation? Well, it's something you have to do. So is not killing George Bush an obligation of mine? No, that's just morality. Why isn't it an obligation? Nothing you did makes it wrong to kill Bush; it always is. But that's not true! We can imagine some life I could have led where it would be right for me to kill Bush. (Am taking the liberty of having Bush have done some things differently in the counter-factual, but that's necessary regardless because what if the new life for me involved talking to him.)
Alright, let's try again. What's an obligation? Obligations are changes or differences in the moral landscape. The idea here is no one alive is justified to kill Bush, so my not being justified in killing him is not a change or difference. But if I agree to meet someone at the park Sunday, that is an obligation because the requirement to show up is different from what other people have to do Sunday. Sound good?
Sorry, nope, that one is incoherent too. Additionally, it's ambiguous, so I'll go over both possibilites.
One possibility is obligations are changes in the moral landscape compared to the average person in our society (can't be compared to everyone, because on any issue where there isn't total agreement of every last person (that isn't us), we couldn't compare). But what's an average person? Mean, median, or mode? Well mean (add all values, divide by total number of values added) is right out. You can't just mix views and expect a coherent result. The mode (whatever value comes up the most) won't work either, because all worldviews are unique. And the median (arrange data on a number-line, count in from both sides at equal speed, and thus find the middle one) won't work either because we can't just line theories on a number-line -- they don't compare that way.
Alright, so we're not comparing obligations to some sort of average in our society. The other possibility is we are comparing to the default. By default, it's wrong to kill George Bush; this is always true unless something happens to change it. Sound good?
Sorry, no. Here we are picking one moral landscape (named "default") to compare everyone to. Any differences are obligations. But society has changed drastically in the last 2,000 years. Could we really have used the same default then as now? How could the 2,000 year old one have mentioned not to kill Bush?
Well, it can't. It would have to say something more like "By default, don't kill innocents." But then to determine obligations, we can't just compare with the default moral landscape, because it doesn't have answers to all propositions. It doesn't have {Kill-Bush=No, Kill-Nader=No, Kill-IMAO=No, etc}
But it's worse than that. Is the default moral landscape supposed to correspond to a default life? If not, How would we decide what goes in it? But if so, what's a default life? The truth is there's no such thing. There are as many ways to approach life as there are people.
Anyway, the point is obligations are incoherent. Not just a little fuzzy and misunderstood, but incoherent beyond rescue. They make no bloody sense. They don't exist.
This doesn't mean you can now cheat on your girlfriend. It just means technically what's stopping you is contained in morality and your choices, not in an "obligation".
But wait, Elliot. What if you agree to meet a friend at the park Sunday, then another calls and wants to do something else Sunday? What do you tell him? Wouldn't you say you had to do something else. (Yes.) And isn't "I have to do something else" equivalent to "I'm obligated to do something else." (Yes, again.) So what are you doing talking in incoherent terms like obligations?
Well I figured out what they're good for! They can be used to express (emphasise if you like) a difference between your view of a moral landscape and that of the person you're talking to. In my view, it's right for me to go meet the first friend at the park. But the second friend doesn't. Thus I say there is an obligation to express the difference in our views.
A close variant is that obligations can actually be used to express the difference between your view any other you choose. For example you might acknowledge an obligation not to cheat on your girlfriend. This is expressing a difference between your view and any view from the class that does allow for cheating.
So to sum up, speaking of obligations is useful to express or emphasise the difference between two moral landscapes (or worldviews, or problem situations, same difference). But obligations don't exist anymore than "bigger" exists.