Cloud computing. It evokes PCs floating merrily in the sky, but in reality it means this: racks upon racks of high-speed servers. And when computing takes up this much space, it needs a landlord. Enter Digital Realty – a property trust that makes the perfect home for would-be data center operators. An early cloud entrant, and one time market darling, it's now facing stiff competition from computing big boys, Amazon, Google and Microsoft.