PG's right Denali - in Wisconsin, you could register at the polls provided you had some form of identification, including a birth certificate or piece of mail with your name and address on it. You could bring your food stamp card, a copy of a welfare check, driver's license, state ID...pretty much anything that had your name and an address on it, even if it was for a shelter.
The homeless and poor have every right to vote (and maybe they have even more of a reason to vote) - but it was just another PR headache in an already muddy election. I have no doubt this is something the Republicans pulled somewhere too, the Dems just got caught on tape.
*Edit - I just looked at yesterday's paper to see what else it said (this is from the Wisconsin State Journal) - they actually went to a specific homeless shelter (the Rescue Mission), and took them in groups to vote (so when the first group got back, the rest of the people could see they got cigarettes after voting, since the Democratic staffers handed them out in front of everyone). It's undoubtedly fishy, most likely not illegal since they can say technically they never promised anyone cigarettes for votes.