Around 17% for me. Some people noticed before that, but most people didn't have a clue until then. Right now I'm about 30% down, and nearly everyone has noticed (aside from my particularly oblivious friend - he's so adorable!).
Hm, I would say about 15 lbs down people started to make comments. I didn't start to see a difference until then either. Granted around 15lbs I dropped about a pant size...so I guess it was noticeable.