There is a site for Certified Dietary Managers where you can find out about certification.
I was Exec Chef in a hosp from 05 until Sept 07 now I'm in a Retirement Center and I must say that Certified Dieticians starting out didn't make much but, from my understanding, if you have any time as an Exec Sous or Exec Chef and you're a CDM you're worth your weight in gold to the industry.
The Registered Dietician where I am currently only works for us 2 days a week but hires herself out for the rest of the week and makes twice as much as I do a year!!! It's a lot of responsibility and you have to maintain a license but it's worth it if your tired of being on the firing line.