It appears that there are several major exchanges. Why is the price USD price 96 on one exchange and 88 on another?
For comparison, Bitfinex currently shows 91.77/91.98, BitStamp shows 91.76/9.99, and BTC-e shows 89/89.033, while Mt Gox shows a last trade at 98.50. Given the hassles which Mt Gox has had processing USD ever since their "temporary hiatus" on 20 June (and their "resumption" and "improved banking" on 4 July, and their "enhanced banking compliance" on 10 July, etc.), it wouldn't be too surprising if additional demand for Bitcoin -- which has no such hassles -- caused the price on that specific exchange to be higher. In other words, if you have dollars sitting in Mt Gox, you might be willing to pay a bit extra for Bitcoin so that you can move your capital elsewhere.
Historically speaking, the size of the difference between Mt Gox and almost anywhere else shot right up around the time of their 20 June announcement.