It's l-o-n-g overdue, but the official car dock/kit (SKU: KPH6786Q) for the HTC EVO 4G is now in stock at many Sprint stores across the country.
Selling for a rather pricey $59.99, the Car Upgrade Kit for EVO 4G (as it's officially called) includes windshield and dash mount options and will put the phone into navigation mode as soon as it's docked. The default car mode is portrait only, but there are some apps floating around out there that will enable landscape use.
Other things to note before paying $60 for this are that the plastic holder blocks the phone's rear camera, the kit doesn't include a charger, and the dock prevents access to the EVO's SD card.
The latter point means that music playback through a stereo system's USB input is not possible. According to dabigcheezze, who posted the first pics and video of the dock, "the phone won't mount the USB because of the way the USB is pinned. It's not set up to sync [or] do data." It can only charge the phone.