I am wondering what cases are suitable to run a system at least in idle mode at 0rpm?
I don't think there's a universal answer to such a generic question.
Just for example, I run since more than two years an Antec Solo with a slightly overclocked Core 2 Duo totally fanless: a TR HR01 as cpu heatsink (ducted to a not working Slipstream), a Quiet Drive - with a Momentus 7200.4 inside - elastically suspended, a fanless overclocked Nvidia 7600GT, all feed by a FSP Zen 300 (ram is four Dominator sticks): the only noticeable source of noise is the DVD burner when working.
If you can run that fanless in a normal case like Antec solo which does not even have top exhaust holes, then any case would be suitable.
That is certainly good to hear, I was thinking about seasonic fanless PSU, one of the I3 with 35W with a Scythe Ninja heatsink, some low power ram and when thats running well, I will try adding a Powercolor fanless 6850, with backup case fans somewhere which I will turn off in idle.
Those small mini pcs are not what I can use, due to the enormous size of the fanless 6850.