I think you need to define a whitelist of allowed tags. I found this tip at http://www.redmine.org/issues/843#note-15, and did the following to allow iframe tags in our instance:
- Add a new file in redmine's config/initializers directory. I don't think the filename matters. I called mine allowed_tags.rb.
- Put this line in the file: RedCloth3::ALLOWED_TAGS << "iframe"
- Make the permissions and ownership match the other *.rb files in that directory.
- Restart Redmine.