This is a great question, I remember reading really good detailed explanation of it by Peter Surda (I believe)

in short, this is the explanation that best answers your question:


at that point, the price became "NOT ZERO"... I don't think it is fair to call $0 a "price" so, I believe you are searching for the moment that it was something...

here is the source of that quote:

http://nakamotoinstitute.org/mempool/the-original-value-of-bitcoins/


btw... the above quote was not from Peter's work, though his 93 page thesis is well referenced and an interesting read so here it is:
http://dev.economicsofbitcoin.com/mastersthesis/mastersthesis-surda-2012-11-19b.pdf