I got my degree in nutrition & dietetics about 11 years ago and here is some info. to clarify.
A Registered Dietitian(RD)is someone who takes a test administered by the American Dietetic Association. In order to take the test you must have a bachelor's degree in nutrition & dietetics from a certified program and complete a 1,000 hour or so practicum. You have to maintain CEUs to remain certified. Many states also require you to be licensed in order to be called a dietitian.
You can complete an undergraduate program and not take the test to be an RD, but all hospitals, some health clinics (i.e. HMO), and government program only hire RDs.
I think that there are nutritionist degrees that are not from certified programs and are for those who do not want to be an RD.
You need to know what your career goals are in order to decide which to pursue. You will need a solid science background and it will probably take 2-3 years to complete the certified program (including the practice hours)if you already have the prerequisite courses.
For more info I would visit eatright.org the ADA website.
Hope this helps! If you need more personal or detailed info, let me know.
P.S. I am currently an RD but have let registration lapse since I no longer work in the field. I may regret this since I will have to retake test if I want to be a "dietitian" again.