Unfortunately, this does not mean turn-by-turn, but XRoad's G-Map is announced for "early November 2008" wich basically is like... um... now ;)
G-Map features offline map data as well as routing, making it a nice alternative to Maps, especially when there would be need to use roaming or you don't even have a cellular connection at all.
Beside that, G-Map has "3D view of intricate major intersections and highway junctions" as well as POI handling.
It seems that in November there will be an US version only, as they say "G-Map for Europe, Central/South America, Asia and other countries and regions will be available by the end of the year." No pricing information is available yet, it "can be confirmed once they have been officially put up in the App store".