Well in the world of small form factor the main problem is that air cooling is less efficient than it otherwise might be. Cooling is heavily reliant on casing shape due to necessary access to vents or proximity to sides for use of passive methods.
I wondered then if mineral oil could be a good solution. The casing shape can be anything really as long as the oil is sealed in properly. All you need to do is to establish some liquid movement around the hot areas, but the normal fans (or bubbles) could do that. Also the weight penalty of carrying liquid around is reduced due to lower case volumes.