Passive PC

[watercooled]

Bear in mind that even boards/coolers advertised as passive may require some airflow over them to keep them at a comfortable temperature - I assembled a mITX server with an Atom D510 board and with no fans in the case, I can't remember the temperatures but the heatsink got hot to touch, as did the ICH chip. And that's a very low TDP chip. A single quiet 120mm fan suspended next to the motherboard in the case brought everything down to little more than room temperature.

I'd consider using a positive pressure case with double filters as suggested above, a single intake fan might be enough for a low power system. And it wouldn't hurt to put a fan on the CPU heatsink as I'm guessing this is going to go somewhere pretty loud anyway.

[[GSV]Trig]

Its not so much the loud but the dust, its in the crappiest dustiest part of the warehouse..

[scaryjim]

I think in terms of dust you're almost certainly best looking at something in a cube case with a large intake fan at the front, with double filtering as Pob suggests. Even a passive case if going to have some convectional air flow which will draw dust through the case, and if you don't control the airflow yourself then you can't be sure which bits to filter. A single large intake fan with double filtering will ensure that all the airflow into the case is filtered. It's a method I'm about to employ on my HTPC, after my enforced rebuild showed that it had accumulated an alarming amount of dust when the only fan was an exhaust...

[MSIC]

Just read this, in case you are still looking:

http://www.silentpcreview.com/article1173-page1.html

[oolon]

Personally I have a Jetway JBC350 Intel Atom N270 1.6GHz with a compact flash, ok I run openbsd on it, however if you want something low powered (12W) and do not need much computing power than this is an option.

[Pob255]

Just read this, in case you are still looking:

http://www.silentpcreview.com/article1173-page1.html

That's a lovely looking fanless case, the form factor would require a little planning to locate it.
I'd suggest having a couple of simple U shaped metal brackets made which could be screwed/bolted to the underside of a desk/table to make a cradle to hold the hold the case just under the table top.
The width of the front would stop the case from being pushed back and out of the brackets, but it could still be slid forward and out to be serviced in any way.

Also being kept off the ground and under the table top would help reduce dust build up.