it depends on the premarket volume before the open and the bid/ask. Typically if the volume is high it will almost always open between the bid and the ask. This is the case for high volume stuff like QQQQ, DIA, AAPL, and RIMM. A lot of stocks have weak pre market volume and wide bid/ask spreads so predicting the open is much more difficult. However, 90% of the time it will open between the bid/ask.