First HN submission I could find mentioning the price: 30 Apr 2011 "Bitcoin hits US$ 4, after being mentioned on CNN yesterday" https://news.ycombinator.com/item?id=2501006

One of the comments "I've been watching the price of BTC rise pretty steadily in the last few days but I can't bring myself to believe that this price range is in any way sustainable. $4 per bitcoin? ..."