Some new information has been leaked by VGLeaks that states that not only are the rumors about the next-generation Microsoft console false, but the system will support backwards compatibility, albeit in a roundabout way.
According to the site, backwards compatibility will be achieved via a add-on device. The device will be similar to the HD-DVD add-on device sold for the Xbox 360 to play HD-DVD discs.
Regarding "always online", the next-gen console (codenamed "Durango") will be always online. However, it will not require an online connection to play games, nor will it block used games.