I ran into the same problem when trying to embed Google map widgets into articles. The solution that I found was to do the following:
- In Article Preferences
(Content->Article
Manager->Parameters), under
"Filtering Options", deselect
the group associated with your user
account -- this will disable the
server-side sanitization
- On your
user account page (Site->User
Manager->your name), under
"Parameters" set "User Editor" to
"No Editor" -- this will prevent the
WYSIWYG editor from stripping out
the IFRAME tag
Now obviously this solution has one glaring problem: you're not going to want to do this for groups the general public will fall into. In my case, it was just for "Super Administrator" so I considered it an acceptable risk.
You can easily toggle the user editor on and off, so if you only have to add such content sparingly it shouldn't be too terribly annoying. The other advantage here is that this is a stock Joomla! 1.5 solution so future upgrades shouldn't break this workaround, and it doesn't require an extension which may impact performance.