If you want to insert an iframe into a Joomla article using JCE Editor Pro (my preferred editor in Joomla) you may need to enable this in the Editor Profiles for the profile you are using to prevent the code from being stripped.
Go to Components -> JCE Editor Pro -> Profiles and select the appropriate profile.
Select the tab Plugin Parameters and scroll down the left hand side to find the Media Support option. It's quite far down but the Allow IFrames is back at the top. Change this to Yes to allow any content to be loaded.
