I have been talking about this to friends lately too, and it is so sweet! DS calls it "Nay-Nay", but I have also heard other toddlers call is Nee-Nee, Nee (just one, not the double Nee-Nee of another toddler), Nuh-Nuh, Yum-Yum, Ma, and Mama Milk. I am so happy that I know so many nursing toddlers!
Oh and SparklingGemini, I hear you on baby words driving you nuts. My son has only Nay-Nay and one other word that are "baby" words. The rest I work with him on and it drives me nuts when he reverts back to baby talk. The other word is so sweet that I can't bring myself to work with him on it. He says "Me-gurt" for yogurt. He totally got the pronoun thing that when I say "you" he says "me" really early and started calling yogurt "megurt" when he was about 18 months old. It makes me all melty.
Ok, back to your regularly scheduled sweet nursing names thread!