When noted economist Chris Blattman mentioned it, my skepticism waned considerably. Surely a man with as finely-tuned a Crap Detector as Dr. Blattman's wouldn't be taken in by a clever internet hoax. Right?

Then Jason Kottke gathered some evidence supporting the existence of a man named Jourdan Anderson, if not the notion that Anderson personally penned (or dictated, as the story goes) the original letter. This lends credibility to the narrative.

Next, I found some insight from that old bastion of fact-finding reliability: The Snopes Message Board. (Remember when Snopes was the first place you went to disprove something like this?) Amongst the chatter, a theme emerges that's been hitherto under-emphasized: the letter is authentic, inasmuch as it was published in a couple newspapers in 1865, but these accounts are ambiguous as to whether Anderson had a collaborator.

It doesn't matter to me whether Anderson himself wrote the letter with a quill and ink and his own five fingers, or mentioned a few broad ideas to be transcribed by the Ken Burns of his day. The important points are:

1. The letter was most certainly written and widely published in the 1860's. 
2. There was a man named Jourdan Anderson, who lived in Ohio after being freed from slavery. 
3. The now-famous letter was written, at least in part, by this man.

Incidentally, this letter was mentioned at MetaFilter in October, 2000 and again in April, 2010.