Apple has returned to the hallowed ground of a $1 trillion valuation, and one of the high points was its spectacular iPad salesApple sold $4.9 billion worth of iPads, it said. The iPad's 22% year-over-year improvement "was our highest iPad revenue growth rate in six years," CEO Tim Cook told analysts in at the quarterly conference call.One of the keys to that was Apple's increasing success in the $4 trillion enterprise market, he said. Businesses spend nearly $4 trillion each year on tech, and Apple has been steadfastly partnering with enterprise tech vendors and adding more business-friendly features to iOS devices to go after this market.Apple has not discussed how much revenue it makes from enterprise sales in years, but back in October, an expert estimated that it was likely aIt's an area that Cook is clearly watching. He said that Apple's strategy wasn't just to sell to more companies, but to "expand our reach and share within large accounts."In other words, he's trying to get his biggest business customers to buy more stuff from Apple, a strategy known as "land and expand."Cook dropped a few impressive stats to note the progress on that strategy, saying iPads are now being used in the cockpits of 450 airlines (he dubbed the iPad the "preferred choice"). That was a subtle dig at Microsoft Surface, which made headlines some years ago when a few airlines chose that device for their pilot cockpit needs.But Cook's real point was that thehave encouraged airlines to put iOS devices everywhere, allowing Apple to sell more devices and also more services."We've been making great progress expanding Apple's footprint beyond the cockpit into the cabin, where more than half of the top 50 airlines have now implemented iOS to enhance the guest experience as well as enable a new use case with mobile point-of-sale," he said.For instance, iPads are being used by ground operations and flight maintenance. At some airlines, they are being used in the cabin, "growing adoption of