You might want to hold onto your 360 if you plan to get an Xbox One when it launches later this year. According to a comment from Microsoft given to Polygon.com, the new system won't play Xbox 360 games. The reasoning sounds very similar to why the PS4 won't play PS3 games: the tech in the new system is too different from the old system to play the old games.
If it follows the same logic as the PS4's lack of backwards compatibility, does this also mean that all the XBLA games won't carry over to the next system either? We'll keep you informed.