Akrasia, both the term and its first instance, are from Plato (Wikipedia backs me up on this ;) ).

Wikipedia's summary is clearer, if less true to the text, than the one I wrote, so, basically, Plato says akrasia is impossible because: "A person never chooses to act poorly or against his better judgment; actions that go against what is best are only a product of being ignorant of facts or knowledge of what is best or good."

As for the weight loss thing, very few people desire "losing weight"; they desire "being fit" or "looking good." "Losing weight" is merely a means to those ends. It would be like saying you desire going to the dentist; almost no one wants to go to the dentist, but they do want to have good teeth, and it's necessary to that end. If you look at the problem this way, it makes more sense, though I agree that eating the cheesecake is not necessarily akratic.