New Mac Mini is 6.5"x6.5"x2". DIN1 size (standard car radio head unit size) is 7.375 wide by 2.25" high. I am wondering why it is so close... ;-)
Mini may be a good media box provided it is quiet. Dustin Mihalik says Mini is quiet but I am not sure since I played with Mac G5 and it was pretty noisy at full load even with all its smart fans and multiple cooling zones. My guess is that Mini is probably as quiet as Powerbook. However, I have no idea how quiet Powerbook actually is when it playing video or audio at high bit rates. My experience with Intel-based laptops has been pretty poor. Before I built my own media box I tried to use old IBM laptop as well as Toshiba 3480 and they both had pretty noisy fans.