This should be an easy concept for most people to grasp. William Blake summed it up pretty good with two of his poems, The Tyger and The Lamb. The real crux of these two poems is in The Tyger, the lines:
What immortal hand or eye
Could frame thy fearful symmetry?

The fearful symmetry is the fact that both evil and good need each other. This is because, in defining good, you need to reference evil, and vice versa. The narrator of The Tyger is a voice of experience, though, and is therefore tainted by the real world. In order to fully grasp the concept, one needs to also read The Lamb, which will provide a counter point. Draw your own conclusions.

The epitome of my...whatever.
I give you the (flawed and horribly broken) proof that good = evil.
Many thanks to Nick for the inspiration.

Prove: For all objects o in U, where U is the universe of all objects, and G is all of the Good objects, and E is all of the Evil objects, o is in G and o is in E.

Proof:
Let x be some object in U.
Also, suppose that x is in E.
Then x is Evil.
For x to be in E, x must provide both pleasure and suffering.
Suppose that y is in G.
Then y is Good.
For y to be in G, y must provide pleasure, but is never easy to attain and hence, causes suffering.
We see that from the properties of G and E, both provide pleasure, and both either come at a cost of suffering or cause suffering.
Nevertheless, suffering is experienced and pleasure is experienced.
The large majority agrees that suffering is Evil and pleasure is Good.
Therefore by the properties of x and y, we see that they have the same requirements for membership.
Hence, we can say that x = y.
Furthermore, since G and E have the same requirements of membership, G = E.
Therefore, for all objects o in U, where U is the universe of all objects, and G is all of the Good objects, and E is all of the Evil objects, o is in G and o is in E.

Please provide any counterexamples or any holes you see in this argument.
I will be happy to collect them.

Counterexamples / holes:
Dru: good and evil are all relative to perspective, hence the point of generalization is moot.  (However, if two different people hold one thing as both good and evil respectively, then this provides strength for the assertion).

- You presuppose that All good things (x) must provide pleasure at the cost of suffering, while all evil things must must provide both pleasure and suffering. Where do you get that from? Those are just arbitrary parameters...