I know there is no teacup designation, but I wouldn't take offense if someone asked me if my dog was a teacup. Those people are just ignorant about the breed. Teacup is more of an adjective than a breed specification anyway. LOL. I think a lot of people use it just as an easier way to describe their yorkies/dogs.. "mine is a teacup" rather than saying "mine is 3 pounds and smaller than the breed standard".
(I don't call my dog a teacup though, to clarify.. haha.. just saying I understand why people do.)
Anyway, even though there isn't a Teacup Standard, I think most people consider 2-4 pounds to be "teacup".. and anything under that is said to be a Micro T-Cup Pocket Microscopic dog.