Hi,
Forums were designed to protect against potential malicious content entered by untrusted users. iframes can be used for malicious content and therefore it is kind of dangerous to allow strangers to put iframes in your web pages. Our protection strategy is not based on protecting user input for a number of good reasons we instead protect output so we use NeatHtml to filter out things like iframe which are not allowed because of the risks.
However if you want to allow such content you can mark a user as "Trusted" from the manage users page. If you want to trust all users then you could do
UPDATE mp_Users Set Trusted = 1
and you could change the default value of the column from 0 to 1 so that new users will be trusted by default.
Hope that helps,
Joe