On the MyPyramid system, potatoes count as a vegetable. A plain baked potato would be at the bottom of the pyramid (the healthiest choice), and potatoes with added fat (butter, sour cream, fried) would be more towards the top of the pyramid in the vegetable group (where things with added sugars and fats are).
Whether a potato really should be considered a vegetable is another thing. Personally, I think it is nutritionally more comparable to a whole grain because of the starch content, but it does have a good amount of fiber and potassium like many vegetables. My biggest objection is that the classification allows for french fries to be considered a vegetable, albeit a high fat one. Because of that, 1 out of every 4 "vegetables" that Americans eat are french fries. Definitely something wrong with that!
The diabetic exchange system puts potatoes into the starch category with breads because of the carb content.
The MyPyramid system isn't perfect, but if we actually followed the recommendations, we'd be in a lot better shape!