Anyone can call themselves a “nutritionist,” but not all nutritionists are registered dietitians. The "RD" credential is a legally protected title that can only be used by practitioners who are authorized by the Commission on Dietetic Registration of the Academy of Nutrition and Dietetics.  Kansas has licensure laws that protect the usage of the title, "licensed dietitian”, or any form or abbreviation related to it, but in some other states without licensure laws, virtually anyone can call him- or herself a "dietitian" regardless of education or training.
A registered dietitian is a food and nutrition expert who has met academic and professional requirements including:

• Earned a bachelor's degree with course work.

• Completed an accredited, supervised practice program.

• Passed a national examination.

• Completes continuing professional educational requirements to maintain registration.

Reference: http://www.eatright.org/Public
Accessed April 11, 2012