The USD ticker was switched to bitstamp. The spread between Mt.gox and other exchanges is getting a bit ridiculous and I don't beleive it to be a fair representation of the current market price. The charts (
http://blockchain.info/charts/market-price) will still use Mt.gox for consistency.
Seems like it would be easy (relatively, given the data you already collect) to calculate a weighted average between all the exchanges you track, which would be a better one number representation of the exchange rate.