iPad mania has hit since Apple's answer to the tablet became available for pre-order on Friday morning. We already mentioned with glee that the mute switch has been transformed into an orientation lock. Now, sales estimates suggest that virtual iPads are flying off virtual shelves as fast as Apple fans can click "Buy Now," and a number of details are surfacing about iBooks, 3G data services, and more.

Of course, Apple hasn't made any official announcements about sales numbers, but that hasn't stopped clever buyers with a Google spreadsheet from estimating how many iPads Apple is selling. Using order numbers matched up to the time of order, estimates range from about 20,000 to 25,000 iPads sold per hour in the hours after Apple raised the curtain. Assuming the majority of sales are the $499 entry-level model, Apple could be grossing about $10 million per hour. If this rate keeps up, it's possible Apple will sell through its first batch of iPads (set to ship April 3) this weekend.

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