I purchased an iPad with the 3g card to try out both at home and in the car.
I already have an 8" screen in the car and may be able to replace it with the iPad. I expect it to be at least as readable as my current Xenarc.
I have also been using the iPhone exclusively in the car for about a year. While my thread on this topic is extensive, here's what I've found out about usability in a nutshell:
1. The screen is too small
2. The interface is unsuitable and downright dangerous to operate while driving
The iPad should change a couple of things in this realm by providing a larger screen and an opportunity for a front end of some type that is easy to use in the car.
The good things are that this is a small computer in a small package with a battery. Like a laptop, it should be easy to mount/dismount which will important for me since I don't own a laptop and want the iPad mainly for couch-browsing. Many of the apps that are available are inexpensive and all quick and easy expansion of its capabilities. I would expect this to continue.
Most arguments against an iPhone as a PC replacement in my other thread focused on screen size, limited storage, and difficulty of expansion. The iPad fixes the screen size issue but it is unclear whether the expansion issue will continue or not. I consider the storage issue moot if you have a network connection.
BTW - with a thread title like this, prepare for haters.