1.) In the original Lincoln–Douglas debates, one speaker would talk for 60 minutes, the other speaker would talk for 90 minutes, and then the first speaker would get another 30 minutes to respond. Obviously that is not the format for the November 5 so-called "modified Lincoln–Douglas debate" between Newt Gingrich and Herman Cain, being sponsored by the Texas Tea Party Patriots, because nobody wants to hear Gingrich bloviate for that long, and Cain would have to start reciting Donna Summer lyrics in order to bloviate for that long.
2.) The Lincoln–Douglas debates revolved around the issue of slavery. The Cain–Gingrich debates will "focus on steps to rein in government spending and fix Medicare, Medicaid and Social Security," three things which did not exist until Lincoln and Douglas were long dead.
3.) The original Lincoln–Douglas debates were free to the public. If you want to attend the Cain–Gingrich debate, though, it will set you back at least $200, or $500 if you want "prime seating" and access to the "Nite Cap" party following the debate. There's also a special $1,000 "Deranged Person" package, which includes the "best seating in the house," a ticket to the Night Cap party, and a photo with the candidates.
So it's not free, they're not talking about the same things, and the format is different. What exactly makes this a "modified Lincoln–Douglas debate," then? The fact that they won't be shouting over each other the whole time? That's just any debate without Rick Perry and Rick Santorum.