It's collating the available sensory input: is the user fidgeting and pacing? Is he/she rocking? Has he/she sat still for too long? Is the noise level too high/low? Is the user being shouted at?

With a light sensor (like the n900 has) you could monitor light levels - is the user skulking in a darkened room? In a bright, fast-moving environment like a busy supermarket?

For the sensory disregulated individual, these things can mean the difference between coping, curling up in a little ball or self harming. A device that could detect the sorts of environments and offer coping strategies could teach people how to manage their own stress. (A dialogue box says "There's a lot of noise: suggest you put your noise reducing headphone on" and the device plays soft music - then opens something like the snowglobe or liqflow) Eventually the user learns to recognise for themselves environments that are stressful and find ways of lowering the stress.