The LEGO Group have advertised the EV3's excellent backwards compatibility with NXT.

As the cables are the same, the EV3 will work with NXT sensors, motors and, of course, LEGO Technic bricks. RCX sensors will work with the aid of [converter cables][1].

Software designed for programming the NXT will *not* work for the EV3 as the NXT runs firmware, but the EV3 runs Linux. However, the graphical software that ships with the kit will be able to program older bricks.

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From their [FAQ][2]:
 
> Q: Can NXT sensors and motors be used with the EV3 P-brick?  
A: Yes. LEGO® MINDSTORMS® EV3 uses the same LEGO® TECHNIC elements and RJ12 connector cables as the MINDSTORMS® NXT, so all your existing sensors, motors and building elements will work  with the new platform, and the software will recognize them as NXT sensors/motors when they  are plugged in. 

  [1]: http://bricks.stackexchange.com/a/713/154
  [2]: http://mindstorms.lego.com/en-us/News/ReadMore/Default.aspx?id=476781