We have already made exactly this mod along with tag based search. :banana:
For phpbb here..
http://www.phpbb.com/phpBB/viewtopic.php?t=359774
We will very soon release a vbulletin version and others.
You can see it in action on our forums here..
http://forums.boardtracker.com/
Also you can see tagged threads in boardtracker here..
http://www.boardtracker.com/search/?...ardtracker.com
You can also use the 'thread tools' there on boardtracker to tag existing threads.
Other tagging (and web2.0

) functionality is coming soon.