I haven't experimented with how section tags (HTML5) display on browsers that don't recognize it. Either way, it's probably not the best option from a semantics point of view. I think a table would be OK for listing threads since there are distinct rows and columns, but not so much for a single thread page where you can use div tags ver easily.
Aye i was thinking that the listings of topics was suitable for as a table and the posts would be divs though i looked though a HTML5 demo site that seems to degrade well. Which uses sections though i think a lot of it is fixed with javascript.