The Kindle DTP publishing
Terms & Conditions are for nonexclusive, irrevocable, worldwide rights. So authors can publish anywhere else that allows nonexclusive rights. This also means that Amazon can publish them in other counties if that want. I think the terms allow Amazon to also "publish" on mobipocket.com (which they don't do), but perhaps not to wholesale through them.
The real issue, though, is where else to publish. Amazon does not give a very good deal to authors (35% of list price), but they do offer a relatively simple path to publication and a large potential market. Perhaps Scribd or Smashwords, or someone, should offer a simple path for Amazon DTP authors to list on their site too. Calibre can automatically convert AZW (MOBI) to ePub and other formats so this isn't rocket science any more.