I am really considering this. Originally, I thought about a Nexus 7 in the car, mainly because it was cheap, small and had GPS. However, with the mini being released, I think I'll go with this option.
Heres my idea:
Buy a cheap Hard Shell from eBay, then attempt to cut it to provide a bumper like case.
Mount this in front of my radio on my dash, allowing for access to the radio if needed. Do this using some sort of bond or sticky pads.
Click iPad mini into place when driving, remove when not driving.
In theory it should work, it just depends on the strength of the pads I use and the weight of the mini. May even try to make a frame to attach over the front the mini to hold in place better.
If anyone knows of a company who make a two part (Front/back case) it would be appreciated.