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: 
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.
Based on my test Raspberry Pi runs relatively cool even when the CPU is 100% loaded for longer periods, so I think there shouldn't be a problem (I did not try loading the GPU though).