I'm not sure what you mean about integrating the poll with forums. It sounds more like a content rating idea is whats needed in forums so that posts can be rated.
I've considerred the ip address as an option for limiting votes. The problem there is lots of organisations have all their users behind a router or proxy so that all requests from the organization seem to come from one ip address so we could be unintentionally limiting the particiaption of members of an organisation. I've heard of blocking an ip address from voting more than once in a time interval. That seems a little better but still not optimal and also more work to implement.
To me I think the bottom line is if you allow anonymous voting you have to take the results with a large grain of salt.