That's a good point to start at; you can adjust it up or down if you're feeling adventurous or want to control the fan noise more.

The small form-factor systems aren't fanless because of the drive, but because they utilize low-power chips with low "Thermal Design Power". Some of the latest Broadwell ultra-mobile chips use 15 W, or even as little as 3.5-6 W for some of the really low-power Core M and Atom chips. More common "standard" power and desktop chips are usually 30-40 W or even as high as 95 W for some server and workstation grade chips.

Higher TDP means more heat, so systems that use low TDP chips may be able to get away without using fans to dissipate heat away from the CPU. They still generate some heat and if you're using one of those you may notice the system throttling itself to prevent damage to the CPU. However, the CPU throttling doesn't take into account the ability of adjacent components to handle the heat, which can be troublesome in tablets, especially when the battery is close enough to get some CPU heat.

I gave my Atom-powered Windows tablet (Asus Transformer Book T100) to a family member, but I was using it with BOINC and didn't notice many heat-related issues, the problem with it was the USB charging interface. It was barely able to supply enough power to run BOINC and charge the battery. Even without BOINC, it would often be draining the battery if the tablet was in use while plugged in--so if you drained the battery and still wanted to use it you had to let it charge a bit first.

On Android devices, there have been some issues with degraded battery performance from the heat load--which is why BOINC for Android has a built-in temperature setting that stops computation when the battery reaches a specified temperature and charge level.

I haven't looked at any of the new MacBook offerings, but most of the older ones had the fan vents hidden in the hinge area and weren't readily visible. I see the new "Retina display" MacBooks have ultra-low power Core M (3.5-6 W) processors, so they probably don't have fans--but the MacBook Pros will still have the hidden vents for their i5 and i7 chips.

Are you referring to a Peltier junction? I wasn't aware that there were any consumer-level products that used those for cooling... Most use "heat pipes" that use a liquid inside a copper tube that turns into a vapor and carries heat energy away from its source.

Many of them use a water-based solution, it depends on the application... https://en.wikipedia.org/wiki/Heat_pipe