How does the Android phone phone it has been docked in a car holder?
I think the HTC Desire car dock can launch Car Home, but how does the phone known it's been docked?
Some Android phones, like the Nexus One, have special contacts that tell the OS when the phone has been docked.
The Desire actually doesn't have these so can't automatically detect that it has been docked. Car Home can still be launched manually.
Another option is to look at an app like Tasker. You could configure Car Home to be launched when certain conditions are met, like the phone is on charge and in a vertical position (so docked). There are many other ways you could use tasker for this.
Typically, the phone knows whether it has been docked by the placement of a magnet on a certain area on the back of the phone. You can therefore create your own cheap dock using a magnet and some sort of stand. Here is an example with the Droid.