Recently, the government has made a requirement for food labels to show how much trans fat is in a product. I have known for years that margarine was not a healthy substitute for butter, due to the partially hydrogenated oils. This article goes in depth to explain the different types fats, their links to heart disease, benefits and pitfalls of those fats. Interestingly, polyunsaturated fats are another one you should avoid, with a couple exceptions (Certain omega-3 and omega-6 acids are essential, and you do need them to maintain good health)
I feel somewhat vindicated that trans fat has finally been given the attention it deserves.