As Nikhil72 pointed out, Google will never offer turn by turn, at least not until the GPS mapping business changes substantially. Google did not purchase the rights from Navteq to use the map data for that purpose, and if they did, Google Maps would NOT be free to all of us.
Someone may offer a standalone application for the iPhone that will do this, perhaps Tom Tom, but until then, I suggest just ponying up for a real dedicated GPS device (ie go get a Garmin). It's honestly the better way to go. I've tried using a phone before as a GPS, and they never work as well, plus when you get a call it interrupts your directions.