Nothing shouts symbolism in the Bible louder than those two trees. The story does not sound like it is taking about two species of flora, and I do not believe in magical trees with fruit which gives either knowledge or eternal life. As Christy mentioned the tree of life is the easy one for many references are made to it. It is wisdom in Proverbs 3; it is the fruit of righteousness in Proverbs 11; it is a desire fulfilled in Proverbs 13; it is a gentle tongue in Proverbs 15; it is the paradise of God in Rev 2; the right to it is granted to those who wash their robes and taken away from any who alter the text in Rev 22.
To sum it all up, I believe the tree of life represents eternal life and a relationship with God. This was available in the Garden and lost in the fall.
The other tree is more difficult. It is not spoken of elsewhere in the Bible. It seems to represent some shortcut to becoming like God in some way. Having knowledge of good and evil is certainly a way in which we could be like God, but this cannot be a bad thing for imparting such knowledge seems to be the aim of all religion. I believe it represents the authority to say what is good and evil for that is a shortcut to being like God which can and has done a great deal of harm. And this is something which is very easily acquired. All you have to do is beget children.