My charging cable glitched once and kicked me into car mode on my HTC Desire Z. When I discovered that it gives me a dial pad when in landscape mode (which it doesn't natively have) I was sold, but couldn't figure out how to reactivate it again. =) When I tried the "Car Dock" app built in, it just brought up one option, launching, which was set to "automatic" and was grayed out.
Anyway, I discovered there's an app on the market called 'Car Mode Control' by 'IG88 Apps' that is a simple toggle that forces my phone into car mode. It works beautifully.