Looks like the cookie the only thing from keeping people from voting more than once.
To everyone here:
How else could you keep people from vote bombing low friction (no required login) sites like this one?
Does it even matter in this case?
Why the hell are people voting NO?
It works because to see a Yes/No it depends on if you have the cookie set, but to actually vote it checks the IP.
So if someone legitimately votes from a multi-user IP that already was used to vote, it'll show a Yes/No, get sent to the vote page, but won't go in–but it'll seem like it did :)