OBG,
You might not have had a chance to check out thseng's link before posting, so here it is again: http://wildernessmedicinenewsletter.wordpress.com/2006/10/26/introduction-to-frozen-mythbusters/

This covers both the fact questioned by PC2K about hot liquids and the heat loss from our head in addition to lots of other good info. I'm not saying if cold you shouldn't wear a hat, I was always under the mistaken impression (as the article states most people believe), that we lose 70% of our body heat from our head.

The cacti info can be found here: http://www.sbs.utexas.edu/mauseth/ResearchOnCacti/introduction.htm

Look under, "poisons".