What lesson do you think Steinbeck is trying to send through Lennie and why?- And evidence from the book would be nice =] - I'm thinking that he's...

He does not represent the disabled.  Steinbeck says:

Lennie was not to represent insanity at all but the inarticulate and powerful yearning of all men.

He is characterized as a cross between a bear and a giant rabbit in the novel.  Steinbeck's novella is fraught with biological determinism, which is part of his pragmatist beliefs.  According to Steinbeck, pragmatism holds that life should be viewed as it is, not as how it should to be.  One needs to live in the moment, reacting to what is happening in front of him based on his life experience and personal judgment, not on religious or moral teachings.  So, Lennie is, more or less, a biological force of desire.

His dreams are as big as his destruction.  He is an embodiment of the American Dream; he  dreams of the farm where he can tend the rabbits and pet puppies and mice to his hearts content.  In the end, though, he represents the idealized self-destructive, self-deluding aspect of the American Dream, the one that is a false promise.

According to Freud's theory, Lennie is a walking, talking "id," the child-like part of one's unconscious that wants instant gratification.  He is a foil to George, the "supego," who constantly tells Lennie to stay away from Curley and his wife, to "let me do the talkin.'"    Curley too is a foil to Lennie and also works from his "id."  So, when the two clash, Lennie wins the battle, but Curley wins the war; in the end, Curley's group "id" (the posse) so threatens Lennie's individual "id" that George performs the mercy killing.

Unlike George and Curley, Lennie is a "big guy," and he represents a physical threat to other men.  So says Candy:

Curley’s like a lot of little guys. He hates big guys. He’s alla time picking scraps with big guys. Kind of like he’s mad at ‘em because he ain’t a big guy.

Lennie is also a tireless worker: he can do the work of two men.  Steinbeck seems to be saying there is a place for men like Lennie, that they can be protected in the labor force.  Can you say union?

So, I don't know if there is any built-in lesson in the Lennie character.  He is so unlike the other characters and the readers that he cannot serve as an archetype.  But, he does represent a part of a man (and here I quote Freud):

...the dark, inaccessible part of our personality, what little we know of it we have learnt from our study of the dream-work ... and most of this is of a negative character and can be described only as a contrast to the ego. We all approach the id with analogies: we call it a chaos, a cauldron full of seething excitations... It is filled with energy reaching it from the instincts, but it has no organisation, produces no collective will, but only a striving to bring about the satisfaction of the instinctual needs subject to the observance of the pleasure principle.