The box will support both Amazon and 3rd-party content. That's a good idea, given Roku, Apple, and other Web set-top makers already support Amazon's content and popular 3rd-party sources such as Netflix and YouTube.
Amazon has reportedly approached media app developers and pay-TV providers regarding partnerships - as Netflix can vouch, the latter group has generally been cautious about partnering with streaming providers. No details yet about pricing; Roku's cheapest box goes for $48 on Amazon, and Apple TV has an MSRP of $99.
Earlier: Amazon reportedly working on two smartphones