From the little I know about of the Kindle architecture, it would be a simple software update to open it up. Right now, Amazon is making a hefty buck off only supporting its own proprietary standards. I'd imagine that Amazon is a smart enough organization that as soon as the business requirements change, we'll see Kindle supporting PDF, epub, etc., but while there is no market pressure, one can't hold it against Amazon from leeching the market as much as they can.