FWIW I consider the Zalman TNN cases massive engineering overkill. They've approached the problem like "This computer dissipates X watts of heat; how can we safely dissipate that without fans?" whereas a simpler (and cheaper) approach is to ask "I can safely dissipate X watts without fans; how can I reduce my computer's heat output to that level?".
All the time that modern PC's put out 100, sometimes 200W of heat, unaided convection simply isn't going to cut it to remove case heat. At 50W it's feasible, at 25W it's a stroll in the park.
I think that is an interesting point. Though I think both approaches have their own merit.
My current project (Working title "the beast") works on the second principle, turn down the watts till you get to totally passive. When i finally get finished i will post in the gallery. I am trying to organise all the resources that I used to help me so that I can post a mini guide dedicated to reducing clock cycles... If I can reduce the research required for the next person on this quest I will get a happy glow. Anyone who needs some help in the meanwhile pm me and I will send you some links.
To link my experiance of your figures on wattages.
My passive rig, has the processor outputing about 29w of heat that with a typhoon cooling it keeps it between 50c to 60c running MCE. I will tweak it further as I reckon I can knock down the temp just a bit more. 25w would be as you say a stroll in the park.
The approach that you consider overkill though I would not discount straight away. My system would not be everyones cup of tea. i wouldn't like to challenge it with a game for instance, 100% processor load for a prolonged period of time would likely create a situation where I achieve thermal runaway. Ok the mobo would hopefully protect the CPU but I'm loathed to test it in the same way I wouldn't test the air bags of my car by pointing it at a lampost.
Looking to the future I may wish to run Vista MCE if it offers a significant improvement over the current OS (Though what that improvement might be I can only guess) and the cases you mention might be a solution.
From now on I also propose a new lingo when referring to CPU coolers.
Pseudo Passive/fanless (PP)= The cooler relies on another components fan to provide cooling ie a case fan or CPU fan.
Semi Passive (SP)= The cooler runs passive at idle but engages its own fan (or a shared fan) when reaching a set temperature or when launching a particular app.
Purely because I came up with the pseudo passive phrase yesterday and like it! Its also better english I believe.
While I am in a dictorial mood, I also demand to be reffered to as I'l Presidente from this day forward... Or I'll have your goat confiscated by my militia.