Would someone please explain the origin and meaning of the myth in which Adam and Eve eat the fruit from the Tree of Knowledge?

It is often considered a post-exilic myth, picked up in Babylon. Some have speculated that the myth was a sort of distant memory of the "easy and carefree life" of food gathering, as represented by Eden, and the switch to the difficult work of agriculture.

You can find an analysis of the phillosophical implications of the tree knowledge of good and evil here:

http://www.amazon.com/exec/obidos/tg/detail/-/0156005514/qid=1102878348/sr=1-3/ref=sr_1_3/102-4898587-0211317?v=glance&s=books

This a very interesting book, I might post some pertinent excerpts later.

One thing in particular I'd like to know is why pre-Christians worshipped trees while Christians have it as a source of evil/sin/misfortune?

http://www.bibleorigins.net/EdensTreeofKnowledgeLife.html

That was most informative, thank you.