I think I found it? Turn by Turn Directions, with only Wi-Fi to cache directions.

Using Bing Mobile App.

You need Wi-Fi though at the beginning to cache the directions and the maps.

I read about this person who in 2008, did exactly that, but he was not using Bing, because it wasn't around then.

You download the bing mobile version on your phone here:

m.bing.com/download

And you download it, then get directions and drive around.

I still need to test it though, he said it worked perfectly.

This was his thread, read it for more info.

It seems like it should work, but you never know.

Offline Maps with Turn by Turn Directions (Wi fi needed) - BlackBerryForums.com : Your Number One BlackBerry Community