I spent quite a bit of time on this tonight, just so you know. So far, we can't find any great solutions in the Facebook developer forums. We can 'see' where the issue is coming from, but it's from Facebook including 2 styles for the same button. If you see the following page (non-Joomla and non-Hebrew, but rtl, nonetheless):
www.abyanboard.com/phpBB2/
The style included directly in the <head> section through the Facebook javascript is slightly different between the 2 pages, and we're not sure why. The actual HTML that is generated is the same.. it's the minor diferences in the CSS that's problematic. We're still researching how it's different, and we've tried the language inclusion that's on that page as well as a few other tricks to get it to work, but nothing yet. It seems like Facebook is simply including duplicate styles, and the can come in a random order (which determines the final look).
We'll keep you posted if we find anything.