Theiphoneblog recently reported a rumor about turn-by-turn coming to the iPhone soon. The rumor says that Apple might be developing a navigation software for the iPhone. But it's just a rumor, nothing more yet.
Anyway, I'm still confident that we'll see turn-by-turn before the end of the year.
In other news, RoadComms "LifeInPocket" is said to be available on the iPhone soon. As "usual", no data is stored locally, making it uninteresting for navigating abroad (roaming costs, you remember?). Beside that and the availability on the iPhone at this time, it seems quite unlikely that it will feature turn-by-turn-navigation on the iPhone.