As a non-native speaker, I found Stephen Colbert’s book title I Am America (And So Can You!) a little hard to dissect. Why so can you? Why isn’t it So Are You? What’s the full phrase that And So Can You implies?
It's a play on titles from self help programs which utilize ellipsis. For example
I lost 20 pounds in 4 days, and so can you.
This is a kind of strange ellipsis, because just tacking on "[lose 20 pounds in 4 days]" doesn't really create a grammatical sentence, but it's something like that. The ellipsis would be clearer if the sentences were
I lost 20 pounds in 4 days, and you can [lose 20 pounds in 4 days] too.
I am America, and you can [be America] too.
And the ellipsis is not so great with "to be", either. Here's some discussion from Language Log:
It follows the form
I <verb> <rest of predicate>, and so can you!
For example, in
I use computers, and so can you!
the "...so can you" means "you can use computers, too." The part where it becomes confusing, and funny, is that the verb is not an action verb, but "am." So "...so can you" means "You can be America too" in this case.
It's a joke on mistakes made with the English language by foreign countries. If you look at the book there are many more jokes about the book being printed in some Asian country and it seems to make fun of engrish type mistakes.