The WYSIWYG editor (tinyMce) used in BE.N doesn't allow certain HTML tags by default.  I've seen this before with the OBJECT tag.  Fortunately, you can tell tinyMce to allow specific HTML tags.  Oftentimes, you have to also tell it which attributes of those tags it should also allow.

In the tinyMCE.ascx file in the admin folder, there's a tinyMCE.init() function call with a lot of parameters in there.  You can tell tinyMCE to allow an <iframe> tag via the "extended_valid_elements" property.  I added that below, does the trick (the part I added is in bold).  You can add more tags to extended_valid_elements also if needed.  It's a pipe delimited list.

tinyMCE.init({
...
theme_advanced_resizing: true,
extended_valid_elements: "iframe[name|id|src|frameborder|width|height|allowtransparency]",
tab_focus : ":prev,:next"
});

Now edit the page, click the HTML button.  Within the HTML, add an iframe – example below:

<iframe src="http://www.retaggr.com/Embed/BrianCarter" frameborder="0" width="360" height="300" allowtransparency="allowtransparency"></iframe>

Now you can add iframes to your pages.  Check out how I added in my retaggr shown below in an iframe: