Good question.
I've heard a few explanations to this one...one of the more interesting solutions is that Adam was with Eve, and the serpent was not just some little snake but really something more monstrous (the Hebrew word Nahash is used which is the same word used to describe Biblical monsters like Leviathan). Basically Adam was intimidated by this thing and failed to protect Eve from it, she was convinced to eat the fruit, and Adam then went along with it like a wuss.
Here's a link that has some of this about halfway down the page: http://www.salvationhistory.com/online/beginner/class1_lesson2_1.cfm
It's pretty much speculation and not the official teaching of any denomination that I know of.