Don Mattrick, Microsoft's head of interactive entertainment, has dismissed the importance of a console having backwards compatibility. Speaking in an interview with The Wall Street Journal, Mattrick said that just 5% of gamers play last generation's games on their new console, so developing a backwards compatibility system for a new console such as the Xbox One is a waste of time and resources.
He said that "if you’re backwards compatible, you’re really backwards", which suggests that all of Microsoft's focus will be on developing a range of current-gen games for the Xbox One. As the unit is powered by a brand new x86-64 processor from AMD, while the eight-year-old Xbox 360 uses PowerPC internals, there is an architectural incompatibility that would make it difficult to code in backwards compatibility.
The PlayStation 4 is in the same boat: like the Xbox One, the new-generation console from Sony uses x86 internals while ditching an older architecture (Cell), and backwards compatibility will not be offered. However, Sony is working on offering older games through some sort of game streaming service, while Microsoft has indicated that they won't be offering this sort of service, at least for now.