Justin Riley has fabricated a really well finished bolt-in kit for the iPad. (wifi or 3g). This particular kit fits into the 2005-2007 Subaru Impreza/WRX and STi. (They all have the same dash).
This bolt-in kit is a 1:1 replacement for the stock dash. Basically just pop out the old one, and put this in there. It has protection for the iPad, and when used with the Apple iPad Dock will charge and deliver audio. (via the 3.5mm cable)
This kit for the Subaru series will be available on the mp3Car Store soon.
Take a look at how it works.