If you're in the UK, you might have trouble finding fanless cases at a good price. I recommend you check logicsupply.com. They have a nice selection of cases and ship internationally but the shipping costs will make a case purchased there expensive for you.
edit: there are people selling cases designed for fanless operation on eBay and the ISK-100 (no idea how good it would be for fanless operation in practice) should be available everywhere
Wow, I was completely wrong so.
I thought it was a good idea to have a big case so air flow would be better...
No, you're right. The bigger the case, the better. But your case needs to be much bigger than a mITX motherboard and be good at transmitting heat from the inside to the outside if it doesn't have enough well-placed holes. Note also that a real tiny case is trouble, even with holes.
If you're OK with a large case (larger than the Q07), I think a regular case could work. Myself, I think it's a shame to put such a small motherboard in a big case when all you need is holes to allow hot air to rise through the case.
You can of course mod a case to allow hot air to rise.
Talking about the MB, do you think Intel D525 + ION2 is better than the AMD Fusion?
In some ways yes but I wouldn't choose an ION except for light gaming. The 2010 Atoms have efficient GPUs integrated to the CPU.
The Fusion CPU are a bit better than the old Atoms. New Atoms boards should be available in a matter of weeks and could be better choice.
The trouble with Fusion used to be the drivers. Make sure you've got adequate driver support in your distro.
What I meant with my comment above is that there are other Fusion boards such as the Zotac which are better and cheaper than the Asus.
Especially if your distro does not fully support Fusion (will you get Flash video acceleration?) and if you don't want to wait for tests of the new Atoms, you might want to use a mainstream CPU.
If you're OK with large cases, you should be able to get by without fans if you were to put a huge heatsink on the cheapest dual-core Sandy Bridge CPUs and get an efficient board for it. Mobile gear would in principle be more appropriate but there's an availability and cost problem as well as a lack of testing.