I'm not able to see how this is different than N!ck's
Improved Thread Preview Hack?
Also it would be very good if you performed a "LEFT JOIN" to post table in thread retrieval query (just like N!ck's hack is doing). Your method of getting info from post table adds as many query as thread count in forum display page (in every visit) so if say there are 30 threads in that page, your query count will increase by 30. LEFT JOIN method would add 0 queries to your forumdisplay so I guess it is more performance friendly.