Just to clear things up, I did search ("poll ip & optionally: vote") before I post this thread!
I went to the poll section yesterday and voted for a few topics.
Again, I went to the same sections, with a different IP, and I was able to vote.
So I guess it's a cookie thing depending on the IP of the machine.
I think it would be better if you flag a user when s/he votes in his profile, untill the poll is dead. So if a user votes from different IP's it would only count the first no matter how many times s/he changes the IP.