That's the question of the moment, thanks to the publication of C.A. Tripp's The Intimate World of Abraham Lincoln, which positst that the Railsplitter swung both ways.
Andrew Sullivan embraces the idea with Whitmanesque enthusiasm and actually raises a far more interesting question:
what's interesting to me is that even if you gloss all Lincoln's male relationships as homosocial or homoerotic rather than homosexual, they still paint a picture that would offend today's Republican establishment. Whatever Lincoln was, he was very at ease expressing love, intimacy, and affection for other men. The last thing he was was sexually prudish. His early doggerel poem about the progeny that results from anal sex with another man--he has the two men married no less!--would be regarded by today's conservatives as worthy of protest to the FCC.
But today's right-wingers are right about one thing. The truth about Lincoln--his unusual sexuality, his comfort with male-male love and sex--is not a truth today's Republican leaders want to hear. They are well-advised to attack and suppress it. They are more closely related to the forces Lincoln defeated than those he championed; and his candor, honesty, and brave forging of a homosocial and homoerotic life in plain sight would appall them. The real Lincoln is their greatest rebuke.
Whole thing here (reg. required).
Update: Here'a Wikipedia entry that includes a--or is it the?--gay Lincoln poem. A snippet:
But Billy has married a boy/
The girlies he had tried on every Side/
But none could he get to agree/
All was in vain he went home again/
And since that is married to Natty