The concept is similar to what I plan on doing to my 2000 ES, however, I won't be paying $2,000 for a Q-PC that isn't even integrated into the car so as to look stock.
If you know a little about computers you can take a look at the motherboard I'm planning on using in my custom built solution. As you can see, it's quite small and can easily be retrofitted into a on or two DIN-sized case (depending on what I throw in there) for easy installation into a car. The plan is to have the LCD screen (6.4" or 8.4" depending on what I can fit in the dash after moving the digital climate controls to the space the 4x CD changer occupies) have a touchscreen interface in order to better facilitate user interaction while driving. I will also be making a little custom launcher application that will place large buttons on the screen that, when pressed, launch an MP3 player, GPS app, etc. If I can work out a decent system of interrupting the MP3 player (perhaps a push button) and accepting voice commands I will add that as a method of UI.
I will not be installing a keyboard as it looks clunky. I will (potentially...dunno yet) be placing push buttons around the display that will emulate keystrokes in order to accommodate input that isn't possible with a mouse. I've also toyed with the idea of putting a keyboard/trackball combo into the fold-down armrest/cup-holder in the back seat for those that really want to use a keyboard.
I figure by using an extra battery as opposed to the main battery in the car I can keep the power somewhat clean and also prevent drain on the main battery. I just have to figure out a way to make the secondary battery charge off the main while keeping it isolated from the power drain of the car. I do computers, networks, etc....not car mechanics.
Anyhow, I'm still in the prototyping phase and don't have any pictures as of yet, but I think you can get the idea.
You should check out MP3Car.com if you're looking for some seriously custom work. While most of the setups are rather "budget" you will find some highly cool layouts. The forums over there is where you find all the how-to info if you want to get one going on your own.
Depending on how my prototype goes I'll take it all the way and market it. As that may be some time from now, if you're antsy that's the place to go for truly mobile computing info.