Currently, the easiest way to extend the TinyMCE editor for your site is to copy

concrete/blocks/content/

into

blocks/content/

in your local directory. Then, open "editor_init.php" in your local directory. Toward the top you should see the following, or something very similar:

<script language="javascript">
tinyMCE.init({
mode : "textareas",
editor_selector : "advancedEditor",
theme : "concrete",
width: "580px",
height: "380px",
plugins: "inlinepopups,spellchecker,safari,advlink",
inlinepopups_skin : "concreteMCE",
theme_concrete_buttons2_add : "spellchecker",
spellchecker_languages : "+English=en",
relative_urls : false,
content_css : "<?=$th->getThemeEditorCSS()?>"
});

Anything listed here can be added to this list of properties:

http://wiki.moxiecode.com/index.php/TinyMCE:Configuration

For example, if you wanted to make iframe a valid tag - so that it doesn't get stripped out when adding the content - , you'd add

extended_valid_elements : "iframe[src|width|
height|name|align]",

Note: don't include any spaces in your JavaScript.