In case you haven't seen this earlier – there is a new version of the Microsoft Bing Maps for Windows Mobile that adds a new navigate option with full turn-by-turn voice support. Fantastish!
The first hit came from Google, who announced back in October that they were adding free turn-by-turn functionality to Android. Nokia followed up with a massive haymaker in January, bringing free voice navigation to over a dozen of their most popular handsets. This time, it’s Microsoft.
You can find it in the marketplace.