We had a cat get stuck up a tree, I found him because of his meow, and he never meowed. Because of the way their claws are made they are good at going up, but not so good at going down. My cat was up a tree for 3 days, so my dad used his tractor, I got in the scoop and he lifted me high enough to reach him.
I'm sure they will eventually get so weak they will fall out of the tree, but I'm not leaving my cat up there for more then 3 days. I'm sure there are alot of people that jump the gun and get upset when a cat is just not ready to come down, but I'm going to say if it's up there more then a full day, it probably hasn't figured out how to get down. Thankfully I never saw my cat in a tree again after that.