Wordpress has (or rather had) an annoying habit -- When you used it as a CMS, the built in editor graciously removed all div tags from your post. I really can't see why anyone in their right mind would wish this. After digging through the support forums, came upon the following:
2.1 version tinyMCE will replace <div> by <p> when saving post, you may change this by hacking file: wp-includes/js/tinymce/tiny_mce_config.php about 24th line, find:
$valid_elements = 'p/-div[*],-strong/-b[*],-em/-i[*],-font[*],-ul[*],-ol[*],-li[*],*[*]';
$valid_elements = '-strong/-b[*],-em/-i[*],-font[*],-ul[*],-ol[*],-li[*],*[*]';
This was the main reason why I was using ChenPress as my editor previously. I haven't yet figured out TinyMCE will handle embedded php in a post... We'll find out soon I guess.
Anyway, thought I would share.
Update -- This hack no longer appears to work in v2.3.1 -- I'm still trying to figure out a workaround.