Another good place to check things out in terms of risk is BME/Risks edition (
http://wiki.bmezine.com/index.php/Category:Risks).
I didn't see anything on cement though. I have read an article in the AMOCK Journal, but that was several house cleanings ago. I'll have to see if I still have it or not.
About plaster causing burns. They do use it for setting limbs, but the limb is usually covered with a sock of some sort before the plaster bandages are applied. Also the bandages account for a good part of the volume of the cast while increasing it's strength. Since there is less plaster the total amount of heat from the exothermic reaction is considerably reduced.
It is also a fact that plaster was used in the movie industry to make castings of actors faces. It has since been replaced by a product called alginate normally used to make dental impressions. Alginate is completely safe and cures far more rapidly than plaster thus making more accurate molds possible. Additionally it is flexible which allows for a rigged positive to be cast directly into the alginate.
More good advice about safety. This is for all of us by the way...
ALWAYS read the label. If it says avoid contact with skin follow the advise.
Further it is important to remember that the manufacture of the product may not have considered you particular application. So DON'T assume it's safe just because it doesn't say it's dangerous.
And always remember that you milage may vary. So test it in small amount. Keep in mind that all skin is not equal... like the genitals for instance. Also you may not get a bad reaction from a small application. So proceed with caution.
And finally finally finally, have an emergency plan if things go wrong, i.e. extraction tools, cell phone, buddy to take you to the ER, etc. etc. etc.