Great Rated 5 out of 5 stars
I'm surprised this hasn't been rated. This is really wonderful. I hope it makes it into the Firefox trunk as Ruby is part of XHTML!
One thing that kept me away from ruby before was thinking that it was only for east Asian languages. But as the official W3C specs indicate, it can be used for annotating any foreign language script, or actually for annotating any text--like a multi-track but for text! (and it is not only more semantic than using a plain "table" element but unlike a table element, ruby text can wrap around--e.g., when the window is resized).
Now East Asian language fans will just need to wait for CSS3 with vertical text support (via stylesheets) to really round out the options.
Tip: check out http://www.w3.org/2002/12/09-rdf-docs/xhtml-math-svg-rdf-CC-flat.xhtml test out the extension (though this sample doesn't wrap the text).