NeoGAF user DieH@rd has found a load of information about the next Xbox, and the Xbox Mini.
He found Microsoft’s Xbox roadmap on VGleaks. The Durango will not support backwards compatibility, but an attachment that you put on the Durango will enable it to play Xbox 360 games. And, more importantly to some the always online function will not affect pre-owned games.
“Durango itself will also always be online like any other device (correct with rumors), But it will not be a requirement to play local content and it will not prevent used games. Putting in an Xbox 360 disc into Durango will prompt the user to attach the supporting device that is sold separately.”
It seems that Microsoft are aiming to compete with both Sony and Apple;
“The purpose of the smaller Xbox unit is to compete with Apple TV, but also provides XBLA and 360 game support which will give it an edge over Apple TV. The rumors of “always online required” comes from the smaller Xbox unit which may not have a disc drive and like Apple TV it would require a network connection and internet to provide any real functionality. It may be possible they will design both consoles to be stackable.”
The report is pretty long but worth the read as it covers everything from pricing to expected announcement dates.