Thread post count will be set at 25,000 and then it will be locked but can be continued on a new thread with the same post count.
Addition #3 (July 1st): Cap the length of your threads This is one of those things that ought to be a setting - even vBulletin is known for choking on large threads, look up rpg.net's motivational poster history. Large threads mean there is a large result set for further instructions in the query to prune from, and if a thread is too disproportionally large, even browsing the thread will result in using table scans instead of the index - a very bad situation indeed.