In July 2011, 1 BTC was trading at above $10 USD. By September, the price of 1 BTC had fallen over 50%, and at the time of writing (October 2011) 1 BTC is trading around $2.50 - $3.00 USD.
In a forum thread one of the pool operators for bitcoinpool claims this is due to some bot forcing the prices down:
Fairuser detected a buy/sell bot on the better portion of exchanges (currently, only Bitmarket (EUR) seems unaffected) that is consistently keeping the value of BTC low while it's generating tens of thousands of dollars worth of transactions across the exchanges at a profit of 2 - 40 cents per transaction. We're still unsure if it's intentionally dropping the price, or if it's just a side-effect of the way the bot is designed.
Either way, the bot is entirely responsible for the price drop from $10 USD to $5 USD.
Is there any evidence of such a phenomenon, and if so, what could be done to stop it?