Don't use a cell phone charger, there's no way to tell what your gonna get. SVOBoy needs to get that verbiage corrected in
his blog.
DO connect to constant battery power, your trips exist in volatile memory and will get lost every time you turn the key if you do not.
I've used a freeduino directly connected to car voltage with no apparent ill effects. The custom versions also use a 7805 with some extra capacitance to help with reliability/transients. There is one person on the arduino forum who uses a 7812 prior to the 7805 but his writeup didn't quite make sense to me, haven't seen the issues he was reporting. If you are able to replace the components on your freeduino with a soldering iron then I'd just hook it up to system voltage myself. This has been an area of much discussion but so far I've not heard that plugging it straight in has been an issue, not to say it won't ever be.