That looks like a search query without a timestamp being passed to it. Do you have a custom active threads link? The search engine has changed a bit in 6.4 and if you don't use the format found in the new links for "Past 24 hours", "Past 48 hours", etc, then it's going to do a full table scan when doing the search and copy to a temp table. Essentialy it will lock the table until it's done so every other query must wait for it to be done.
I've seen this on 2 other upgrades to 6.4, both having a custom link that points to recent posts that was not updated to the new 6.4 format.
