I bought my TyTnII....
...purely so that I wouldn't have to buy a separate PND. It's safer not to leave a PND in the car (I take my phone with me) and it's smaller lighter and cheaper than having two standalone devices.
I suspect a lot of people will begin to do this, now we're seeing GPS-enabled phones with larger touchscreens. If you have a Touch Diamond, Omnia, Nokia etc with GPS built-in, why would you go to the trouble of having a separate GPS device?
Unfortunately, this is the biggest problem with the iPhone and Android. The former is unlikely to get TomTom (or similar turn-by-turn navigation) and so far Android doesn't look like it'll have one soon. So I'm stuck with crappy WM devices. :-\