Simple. They know by serial number. If bought at Apple they can retrieve the date bought, and obviously within a year of the iPad being released (until April 3rd, 2011) they will know that EVERY iPad at that point will still have a valid warranty.
For 3rd party retailers such as Best Buy, when you purchase your iPad is not when your 1 year warranty starts (through Apple). That warranty starts when the 3rd party retailer purchases the units themselves from Apple.
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