Well, for starters, you'd probably want to do a quick lookup on the email address to see if it's registered in your forum. Something like:
SELECT `user_id`, `username` FROM `phpbb_users` WHERE `user_email` = '[color=blue][i]User Input Email[/i][/color]' LIMIT 0,1
Then, if it comes back with a user ID, you know it's a valid user. Then you just need to insert the content into the proper topic table(s):
INSERT INTO `phpbb_posts` (`poster_id`, `poster_ip`, `post_time`, `post_username`, `post_subject`, `post_text`) VALUES (
'[color=blue][i]user_id[/i][/color]', '[color=blue][i]$_SERVER['REMOTE_ADDR'][/i][/color]', NOW(), '[color=blue][i]username[/i][/color]', '[color=blue][i]User Supplied Subject[/i][/color]', '[color=blue][i]User Supplied Content[/i][/color]')
That should insert it into the posts table, then you just need to use [man]mysql_insert_id/man to get the last insert ID (which incidentally is the post_id 😉 and insert into the topics table the new topic:
INSERT INTO `phpbb_topics` (`forum_id`, `topic_title`, `topic_poster`, `topic_time`, `topic_first_post_id`, `topic_first_poster_name`) VALUES ( '[color=blue][i]Your specific Forum ID[/i][/color]', '[color=blue][i]User supplied Subject[/i][/color]', '[color=blue][i]user_id[/i][/color]', NOW(), '[color=blue][i]Result from mysql_insert_id()[/i][/color]', '[color=blue][i]username[/i][/color]')
That should be pretty-much about it. That's based off of phpBB 3.0.0 though, so it may need to change for 2.0. But those are the three queries you'd need to run in order to allow users to "create topics" in a specific forum via your web form.
Now, if you're going through all that trouble, wouldn't it be easier to just link them to the "Create a New Topic" section for that specific forum? So instead of filling out a generalized form, they could literally create their own topic, and if you subscribe to that specific forum, you'll be emailed every time there's a new topic or post 😉 Kills two birds with one stone really. And the link would be simple enough:
http://www.domain.com/phpBB/posting.php?mode=post&f=64