This error appeared in my site too. I checked the default-filters.php file and I came to know that this error is due to a line added by widely spread Conficker worm. To remove the error from your site follow these steps:
1) Scan your PC for existence of Conficker worm. And after cleaning it access your ftp else possibilities are that it might affect other files too.
2)After cleaning your system open default-filters.php in a text-editor and scroll to the bottom of the file. There you will be able to see a line like
add_action('<iframe>...
Delete this complete line. And save the file and re-upload it to your web server. Your site will start working.
I recommend that you check all your files for the existence Conficker worm.
