You would have a seperate table with the polls in, if the user wants a poll you would add a new line to this table, then use mysql_insert_id(); to retrive the ID and store that in your threads table under a "polls" feild.
When the user veiws it you would grab the poll feild, if it's zero, then the user doesn't want a poll, otherwise retrive it and display acordingly.
Voting is a simple matter of an update query.
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