I would be very careful with allowing users to post iframes indiscriminately. it I an opportunity for phishing attacks, malware distribution, etc. For example, an attacker could make a page with a login form looking just like the one on your site, embed it in a post, and make it look like login is required to see the post. It may trick quite a few people to reveal their passwords.

if you know of a website that has a custom iframe allowed for public posting, please share and we will review.