I don't think the rep votes are keyed by IP address. If you view the HTML source, you can see a new field named 'vc' in each of the rep voting boxes. I'm going to go out on a limb and guess that 'vc' stands for vote checksum....

It appears that each ballot is being tagged specifically for a user, and if another user attempts to use it, they get flagged -5 points.

Update: I was refering to rep voting on nodes, not the polls. Rereading the original question, I see that it was probably about the polls (especially since the poster doesn't have enough XP yet to have even seen a rep vote ballot)... I tweaked a phrase or two to make that more clear.

-Blake