from http://wordpress.org/support/topic/76086

Tiny MCE as configured in WordPress 2.1 wipes away divs and replaces them with p tags.

This can be fixed by doing the following:

1-Open your TinyMCE config file in an editor. The file is located at:
wp-includes/js/tinymce/plugins/tiny_mce_config.php

2-Replace Line 25 with the following but on a single line:

$valid_elements = ‘#p[*],-div[*],-strong/-b[*],-em/-i[*],-font[*],-ul[*],-ol[*],-li[*],*[*]’;

3-Save the file back to your active WordPress installation, overwriting the existing tiny_mce_config.php file.

4-In WordPress admin, the editor will now allow divs.

You may have to refresh your editor page.

As always, copy your original tiny_mce_config.php file to a safe place for backup before editing the original.

Allowing divs

Tiny MCE as configured in WordPress 2.1 wipes away divs and replaces them with p tags.

This can be fixed by doing the following:

1-Open your TinyMCE config file in an editor. The file is located at:
wp-includes/js/tinymce/plugins/tiny_mce_config.php

2-Replace Line 25 with the following but on a single line:

$valid_elements = ‘#p[*],-div[*],-strong/-b[*],-em/-i[*],-font[*],-ul[*],-ol[*],-li[*],*[*]’;

3-Save the file back to your active WordPress installation, overwriting the existing tiny_mce_config.php file.

4-In WordPress admin, the editor will now allow divs.

You may have to refresh your editor page.

As always, copy your original tiny_mce_config.php file to a safe place for backup before editing the original.