It’s caused by WordPress’s initial setting that removes iframe tags from your posts automatically.

Here’s an explanation of how to paste iframe tags onto your posts.
I tried this method in WordPress3.1.1.
(CAUTION: This method is your own responsibility)

METHOD:
Open the file
wp-content/themes/<your theme directory>/functions.php
and add the below code at the end of the file.

function extended_enable_elements($init) {
$init['extended_valid_elements']="iframe[src|width|height|name|align|style|scrolling|marginwidth|marginheight|frameborder]";
return $init;
}
add_filter('tiny_mce_before_init', 'extended_enable_elements');

Actually it may not completely show the frame in the view of “Visual-Editor”, but enough is shown in the “Preview” window.

If anyone who can find a way to show IFRAME in the “Visual-Editor” of WordPress then comments would be helpful.

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