How can I change the voice used for the Navigation app on Android?

I love the Navigation app on Android. The voice it uses is fine, but I'd like a little variety. I'd settle for it using the British voice that's available on the phone. However, I've changed the Text-to-speech setting on the phone for language to British, but it hasn't changed the voice used by Navigation. There is not a setting in Navigation for the voice. Assuming it uses the "locale" setting, my only options on the Droid are "English" or "Español".

Am I stuck? Does this require root?

In case it matters, I'm still currently on Android 2.2.