I have the atom in my car and it has been great. So much that I'm building a mini PC for my son using mostly the same setup. I also have it in the dash.
the sound card to the amp as short as possible, and separate them from other wires.
Also, you can only use the headunit wiring for powering the pc (but it isn't the best idea),
as the original speaker wiring is a crap.
an other (half-box) next to it for the connectors, with a thin waterproof film on them,
and with some drying agent (silica gel) inside.
Also, It's almost impossible for back seat passengers to kick the pc my mistake,
as the seat base is 50x50cm (at least), the pc is about 20x30 with connectors,
so they have to get deeply under the seat to reach the carPC.
(but yes, kids can be creative)
Not a problem with an SSD anyway.
Check my worklog:
Corsa + Atom + Gentoo Linux + 9" capacitive touchscreen
Lord of the boards: DFI CP100-NRM
"Or you can try Ubuntu, but than don't tell everyone you are using linux,
because it's just a secret unreleased prebeta of Windows 3829" :P
mount it where ever you feel that it will fit best-- for trouble shooting, and connecting everything in the car, i think it is easier under the seat, but see certain advantages of having it in the dash, as long you run new power/ground wire (i blew 3 main accessory fuses trying to hook a dc-dc to the factory wiring...)
for the atom processor-- they are good for basic computing-- don't expect to be doing any audio/video editing, or use it for a media server, but they work good as a basic, low cost, fanless computer.
my last carputer board, a intel 945gclf has been resigned to home computer use after not having enough processing power for all my audio setup (running audio programs to control the sound). and has been working flawlessly since the swap.
My point was that the reasons you listed were not valid reasons to discount a dash install. There may be space limitations depending on the vehicle and the hardware being used, but like I said, if you can put it in the dash, put it in the dash.
For many people the headunit wiring is fine. I ran one 12 AWG wire from the battery for constant power top the PSU. Ignition switched power should be in the stock radio wiring or at least somewhere near by in the dash (I don't have any appropriate switched wiring under my seat). For most people (>99%) the original speaker wiring is fine. I could give you a detailed explanation, but we are already starting to hijack this thread.
just wanted to add, i'm running a Zotac ION Atom N330 board, fanless in a VoomPC 2 case currently under passenger seat (considering mounting in trunk). Performance is great (running Win7 embedded + testing out Centrafuse 3.0). Highly recommend it.
EWF, HORM, MinLogon on XP.
Zotac ION Atom N330, 2GB low-profile RAM, M3-ATX
Win Embedded Std 2011 RC
OCZ Vertex Turbo 30GB SSD
Lilliput 629 Transflective, WRX Screen Mount
BlueSoleil BT, i-Blue GM-2 GPS, DirectedHD Radio, Andrea Mic