Estimates from Friday's iPad pre-orders suggest that 50,000 iPad units were pre-ordered in the first two hours with 90,000 at the six hour mark. Members of Investor Village's AAPL Sanity Board put together a spreadsheet to estimate the number of pre-orders based on order numbers.
The results as pointed out by Fortune's Apple 2.0 blog say that after two hours of iPad availability Victor Castroll announced "51,000 orders in two hours" on the board.
The estimates were made by looking at two order numbers 30 minutes apart and accounting for the estimated 15,000 online orders that run through Apple's store in a typical day.
After six hours of iPad pre-orders Castroll estimated that orders were up to 91,000.