I was having the same problem, but it has switched into car mode a couple of times now.
My estimate is that it seems to work about 60-70% of the time.
I called Verizon when I couldn't get it to work at all, she grabbed an associates phone and it worked. I then put my phone in my wife's car dock and it worked. I assumed I had a bad dock.
Try putting the phone in the car dock without the dock plugged in and see if the phone switches to car mode. Using this method it has switched every time. I understand this is not ideal if the dock is on your dashboard or windshield but it may hold you over until an update shows up that fixes it.
There is an app in the market that will switch the phone into car mode, I think it is actually called "car mode" it will make the dialer go to landscape, but as far as I can tell, nothing makes the home screen rotate anymore.
I switch to ADW Launcher when I plug it into the dock, then I activate car mode with the app.
This gives me landscape mode on the home screen, app drawer and the dialer. Not as easy as just plugging it into the dock, but works for me.
