The car mount that is made for the BIONIC works with both regular and extended battery. However, the mount is very short, sometimes making it get in the way if you're taking pictures / making video.
The old Droid car mount and Droid 2 car mount's had an extension that swiveled in 2 directions only, but the extension puts it much further out from the base, and it looks like it will allow you to take pics without having the base get in the picture. The actual phone holder from the BIONIC car mount fits the ball joint on both older mounts as well, a major plus.