What you are reffering to is speed sensitive volume controls. Ford and GM have had it for a few years now. Neat feature. Ford's is much better than GMs and has a lot more user adjustment built into it so that the radio compensates for road speed according to the user's tastes

Anyway, nice to see Toyota getting into the mix, but I'm not aware of it being available on the Prius.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)