Question: Is it possible to reliably run a Pi in an airtight (watertight) case or is some form of ventilation required for cooling? I'm assuming it doesn't chuck out too much heat, but am wondering whether running it indefinitely this way may cause issues.

Answer: 
There are **no ventilation or cooling requirements.**

This has been verified by an RPi admin [here](http://www.raspberrypi.org/phpBB3/viewtopic.php?f=2&t=1701).
Yes, under usual circumstances it should be possible to run the Raspberry Pi in an airtight case. I have measured some temperatures confirming that there is amply thermal headroom.

For reference:

* 22°C room temperature
* 42°C idle

I ran [sysbench](http://sysbench.sourceforge.net/docs/#cpu_mode) to stress the CPU:

```
sysbench --test=cpu --cpu-max-prime=20000 run

```

* 45°C 20 s
* 46°C 2 m
* 47°C 5 m
* 48°C 12 m

Since the temperature did not seem to raise any further, I put a paper hood over the CPU to simulate the "airtight case scenario".

* 49°C 20 s
* 50°C 2 m

After another half an hour there was no further increase of the temperature.