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.
If you are using PHP please use the [PHP] and [/PHP] forum tags for highlighting...
The same applies to HTML and the forums [HTML][/HTML] tags.