As a silent pc enthusiast (slightly obsessive at that - even my gaming computer is just about as passive as it gets - 2 slow 500 rpm fans) I have always thought it'd be interesting to see a large custom passive heatsink on the ps3 slim.
I find the ps3 fan noise (especially on the slim it is slightly more upsetting than the ps3 fat) to be fairly upsetting when watching a movie - if i were to guess it sits around 25db during a movie.
I figure if thermalright can make a passive heatsink: http://www.thermalright.com/products/?act=data&id=95&cat_id=27 for the hot i7 processors it should be even easier to make a passive heatsink for the ps3. It could also double as a case.
An example of a heatsink case is the HFX classic: http://www.hfx.at/index.php?option=com_content&view=article&id=233&Itemid=54.

My current solution to keeping ps3 heat and noise down is to have it inside an insulated cabinet outside the room with a long hdmi cable but wouldn't a ps3 slim with a big giant heatsink be quite the showpiece? Alternatively one could do the xbox 360 which even in its newest iterations is still a fairly noisy machine.