I'm ovo-lacto who eats certain shellfish, and I have been for at least thirteen years. I have finally evolved into a philosophy about it -- I think it's hypocritical to ask someone else to kill and clean any animal that I wouldn't be able to deal with myself. Pretty much just leaves me with mollusks and shrimp (I really only eat them at restaurants, but I know I could deal with it).
My dad has evolved a simplified explanation for new people -> "she won't eat anything with a face." I think it works.
I'm interested to find out if there are of the diets like South Beach that have worked well for vegetarians. The emphasis on meat/fish protein has honestly been one of the major things holding me back from trying one of those diets.