Supposedly, the recently released patch 1.31 for Borderlands -- otherwise known as the one that added Steam achievements -- contains files that explain, in a considerable amount of detail, what the game's next add-on pack will entail.
A dude on the Gearbox forums by the name of Legendrew found the score, and proceeded to post the heck out of it in this thread. Seriously, he went all out. If you're worried about potential spoilers, I'd advise against going in too deep. I have read enough of it to know I don't want to see any more.
So yeah, without getting into specifics, this sounds like the best Borderlands add-on yet -- if it ends up being legit, that is. Based on a lot of the language used in the post, I'd say it certainly matches up with the game's particular brand of humor.