I didn't have to have my doctor sign anything, but I did tell my doctor and he said it would be fine if I wanted to do the diet.
From what I understand, you need to get your doctors permission if you want to do sole source, and have a certain BMI (not sure what that is). If my doctor would not have agreed, I planned to do sole source plus until I got to that BMI.
Anyway, I was really nervous before going to meet my CDC, as I hate meetings like they have at weight waters etc, and with it being 1 on 1, I thought it would be worse..but actually, it was really relaxed. I was nervous about standing on the scales infront of someone else too, but thats what shes there for! Now I look forward to going each week.
Good luck!