Jeff - I only read a few of these posts here, but I think deer definitely learn to look up once they bust a hunter, and will learn from other deer, On my food plot i have to move between 4-5 different trees because the mature does walk to the edge and look in the tree tops, especially the ones I hunt frequently.... Tonight i had a young doe looking in trees, and a squirrel almost had her spooked...

I've learned though that if you pick a tree w/ enough cover and you dont move they will look at you and not be alarmed, they need a double trigger, either scent and blob in tree, or blob in tree that moves...