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9to5Mac cites an "Apple Store source who has been reliable in the past" for a speculative report about an announcement that day. "Naturally, this points to an iPad 3 launch on the Friday just nine days after tomorrow’s announcement," according to the report, which also notes that an Apple Store in London at Harrod's and another in Houston are set to open that day.
We're assuming "iPad 3" here means "iPad HD," in which case, the sales date would make sense. There was also a nine-day lag between the announcement of the iPad 2 on March 2, 2011 and its introduction to stores on March 11. If things go the same way as last year, you also won't be able to preorder the iPad before it hits retail stores.