This is the NAT Virtual Interface on ISR routers.
One of the cool things, by virtue of having one of these around, is that you no longer need to worry about your "inside" or "outside" locations... If you have a lot of complex NAT this is very handy.
Now you just need "ip nat enable" and the NVI will take care of the rest.
A. Cisco recommends that you use legacy NAT for VRF to global NAT (ip nat inside/out) and between interfaces in the same VRF. NVI is used for NAT between different VRFs.
I am trying to find out if NVI is now the recommended way for doing NAT configuration in all cases.
Very cool feature, just found a link on cisco's site with more config examples.
Going to try it out.