Some thoughts on front end UI
For starters, what makes CarPC UI different from desktop/laptop UI, or cell phone UI?
0) Generally, the user should be focused on driving, not on operating the CarPC
1) The user's eyes should be on the road as much as possible
2) The user's hands should be on the wheel as much as possible
3) Typically the screen is 7" and is an arm's length away (this varies of course)
Keyboard and mouse are OK for system administration but due to #2 they should not be needed for anything that happens while driving. So the touch screen is the primary means of input. UI elements on the screen (buttons, text, etc) need to be relatively large so that they can be identified at a glance and manipulated at a glance. If the driver needs to focus on the screen and use fine motor control to touch the screen in exactly the right place, that's bad.
A small screen with large UI elements means that each 'screen' of the UI should have only a few elements on it.
Tasks that the user does frequently should be accessible with as few gestures as possible. Tasks that the user does rarely can take several gestures, and can involve screens with more/smaller UI elements, since presumably the user can wait for a stoplight.
The most frequently used things should always be possible with one single touch - things like volume, mute, track-skip, and a 'back' button for general UI navigation. Probably nothing else. So let's set aside the left edge of the LCD for a vertical strip that has those controls on it.
At the very top of the strip, a 'back' button (similar to the back button you get in browsers and elsewhere in regular desktop/laptop UI). Below that, a vertical scroll-bar-like thing for volume; wherever you touch it, that's your volume level. Put a mute button at the bottom of the volume slider. Basically the mute button just gives you a bigger area to stab with your finger when you want low-to-no-volume for whatever reason. Below the mute button, have a little bit of unused space, so you won't be likely to bump the mute when you were really reaching for the controls below it. Below the whitespace, have << and >> buttons that act as track-skip when listening to media, or radio station selection when listening to radio.
(I could maybe be convinced to put a 'home' button under the 'back' button. Maybe.)
The buttons on that strip of UI should never change. Those are the absolutely-most-frequently-used things, so they should always be one click away, and keeping them always in the same place means the driver will soon be able to use them without even taking eyes off the road.
The rest of the UI:
Main screen - has big buttons for each of the major features. Major features would be things like: media player, radio, OBD2, phone, navigation, etc. The exact list must be user configurable, since different users will have different ideas about what's essential. The main screen should also have a "More stuff" button to access features that don't get used often enough to merit their own big button on the main screen (anything having to do with configuration, for example).
Pressing the media player button would lead to a screen with a list of playlists. I rarely want to pick songs while I'm driving - I mostly just want to pick a mood, and then hear songs that fit that mood, without me taking any more action. This screen would also have a 'browse' button that would go to a music-browsing screen.
Pressing the radio button would lead to a screen with a list of radio stations, and a button that allows tuning a station directly.
Pressing the phone button would lead to a screen with a list of cellphone contacts (names and numbers) and a button for dialing with a keypad.
Pressing the navigation button would embed iGuidance or something, leaving the left-edge back/volume/mute/skip UI in place. I haven't got any bright ideas about improving nav software, but that's OK because there's not a lot that can be done to customize it. But that's OK because it works pretty well as-is. :)
Note that the media, radio, and phone screens are all very similar. A big list, to access the most frequently-use feature, and a button that leads to UI for less-common and more-detailed stuff. So all of those screens should look very similar too. I'm picturing a big listbox on the left (but not obscuring the volume/mute/etc strip of course), with an iPod-like wheel next to it, and below the wheel, a "select" button, and a button to browse files / tune radio / dial phone.
Pressing the browse-media button would lead to a similar-looking screen, but with a row of buttons across the top to browse by artist name, album name, or song name. Below that, the familiar listbox, scroll wheel, and "select" button.
Anybody want to build this? I'm can't promise anything in return other than my gratitude. And perhaps the gratitude of others here... who else would use this if it existed?