Some links to articles on the subject:

Why Hitler Was Not A Vegetarian by Rynn Berry

Hitler's Vegetarianism by Roberta Kalechovsky of Jews for Animal Rights

A very brief summary of the conclusions in the above articles:
At various times Hitler reduced the amount of meat in his diet, primarily for medical reasons. Sometimes he reduced it a lot but the evidence shows that he never stopped eating meat completely for any significant length of time. Unfortunately many meat-eaters, including historians, use the term 'vegetarian' incorrectly to describe someone who simply reduced their meat consumption. The confusion was compounded by Goebbels, grossly exagerating Hitler's 'vegetarianism' for propaganda purposes.

See the : History of the Vegetarier-Bund Deutschlands for more details of how the German Vegetarian Federation closed during Nazi times and re-formed in 1945.