Greg of Futility Closet writes about sentences composed entirely of one word. I think these are rather arcane, though. My favorite is Tagalog, where an entire conversation can be composed entirely by one syllable, and it’s something that has more common usage:
1: Bababa ba?
(Translated: 1: Is it going down? 2: It’s going down. — in reference to an elevator)
In fact, an entire article on Wikipedia is dedicated to homophonous phrases.