[Xenon]

Well, you can ignore the headers warning.

this will be on every explain=1 page.
Really nothing to worry about

As for the second query: well, you can set up an index for the dateline column of the post table, but apart from that there isn't much you can do.
joining the post and thread table is always slow

maybe change the query into:
[sql]SELECT count(*) AS posts
FROM post
INNER JOIN thread USING(threadid)
WHERE post.dateline > '1077204189'
AND thread.forumid NOT IN (16, 54)[/sql]

it could speed up it a little bit.