not to destroy anybody's hopes, but i doubt that we'll see a port of the google turn by turn navigation.
first, you need the source code. android is partly open source, yes, but i'm sure google keeps this one closed to have a competitive advantage. i searched a bit but couldn't find any licensing info on this. maybe someone has better information?
second, porting from java to c/c++ is a bit more involved than just having to sort out memory management. this application might depend on many dalvik-specific classes, which would have to be ported as well. you'd be opening a pandora's box of additional dependencies which would have to be sorted out...
it's probably easier to write a front-end application to maps.google.com which implements turn by turn navigation and relies on the google maps API.