You may or may not already know this... or even care.
But i just wanted to let you know there is already applications (some for free) that make it so simple taps, or swipes can change songs/volume. Good stuff, I use this when driving all the time, with my iPhone though.
I think an iPad would be sorta large for in-car??
I didn't know, but I do care!
My main gripe with using the iPhone for in-car use (I have a 1st generation one) is the screen size. Specially for navigation apps, which I don't use, but want to.
I have a problem with my eyes and my contact lenses don't help much anymore. So, I need a bigger screen! I still can drive, but can't read small street names on a screen.
My Honda has a USB port that I connect my iPhone and I can control it (the iPod application only) with the buttons on the stereo or steering wheel. It's a bad interface, but the buttons are nice and big and I don't have to hunt for them while driving.
The iPad is bigger than what I need and it would be difficult to find a good spot for placing it in the car. So, you're right: the iPad is too large for in-car use.
Still, I am not buying a 7-inch tablet from another manufacturer. I use Apple stuff since the 1980's (Apple II) and got my first Mac in 1995, after using an Amiga for some years. These days, I don't trust anyone but Apple for my computer stuff.
I just want a big-screen GPS navigator and a music player when in the car. When I get to my destination, I can browse the web, show videos to my customers and prospects and all the other stuff that I can do with the iPad.