We're all talking about mobile CPUs and chipsets, aren't we? I mean ION is a mobile chipset, just like the 945 or the NM10. Atom 'N' stands for Netbook, D for desktop.
I don't believe convection works better when the board sits vertically in the case, I trust more the horizontal position.
Regarding the Antec Skeleton-90 case, no I don't really want to buy one

My daughter would put something in there

Regarding the 3 different setups you are talking about:
Atom N270 + Intel 945 chipset: the N270 is only 2.5W TDP (great!). Problem: no PCI express.
Atom D510/D525 (pineview) + NM10: Problem 1: 13W TDP, it's fine but the need for a fan is more likely. Problem 2: PCI express 1x or 4x only (the Asus AT5 board has an open-ended 4x slot accepting 16x cards, but it actually runs at 1x speed only - the config can either be 4 x 1x or 1 x 4x).
Atom N330 + ION: The N330 is 8W TDP. You have a real PCI express 16x bus and slot. This is the configuration I am interested in. Seems like a pretty good compromise. The AT3IONT-I Deluxe board from Asus seems fairly good.
Don't know about configurations with a N450 or N550. I don't believe that heat/power wise they will be more interesting than the AT3IONT-I board.
Here is what I am going to do:
I'm going to go for a AT3IONT-I Deluxe board, the Deluxe version has an external 90W power supply as I said. For the case I'm going to buy a SilverStone SG05:
http://www.silverstonetek.com/products/ ... 5&area=usaUnlike the Thermaltake Element Q, the SilverStone has a 120mm front fan that I can set up to kick in when the temperature is too hot. Just to be on the safe side!
I am assuming I will be able to replace the fan with a Noctua.
One last question:
The SilverStone has room for a dual-slot graphics card, in case I want to upgrade in the future. But 90W is not enough power to run a fast graphics card. Do you know if motherboards with 19V DC input can have their external power supply upgraded to something more powerful?
Here is an external 19V 120W power supply, not sure if this would work with the AT3IONT-I. I don't know if the plugs are compatible:
http://uk.farnell.com/xp-power/aml120ps ... dp/100574430W of additional power is already offering a bit more choice regarding graphics cards (a GT 430 card for instance, still passively cooled of course)