From what I gather from this discussion page at Snopes.com, it seems the basic facts are these:
- There was indeed a man named Jourdon Anderson who lived at this time; his grave is not far from the spot on which this letter purports to have been written.
- It was printed in the Cincinnati Commercial and reprinted in two other papers at that time (click image to enlarge). So it is old.
- According to one of the reprinting papers, the Cincinnati one declared it "to be a genuine letter from a freedman to his former master."
- Copies of the letter in Anderson's hand evidently do not exist, nor do copies of the letter from Col. P. H. Anderson, to which he is purportedly responding.
- I think that someone who has had the life that Anderson ascribes to himself would have been illiterate, or nearly so.
- But I see another copy of it, from a book published in 1865, in which it is described as "[w]ritten just as he dictated it."
So you can circulate this marvelous letter with a clear conscience. At the very least, it is clearly a contemporary document, a far cry from the familiar version of "Chief Seattle's Speech," which was largely concocted by environmentalists more than a century after the event.