I run a site (WP based) which is going multilingual. After some research, I didn't found any plugin which satisfied my requirements, so I thought I should swap the CMS.
I've been playing with Drupal (and its module "i18n") and Joomla/Mambo (and its "Mambelfish" add-on). Although both systems are really powerful --much more than WP--, I feel much more comfortable using the WP admin interface.
Plus, none of the aforementioned add-ons works the way I like. So, now I'm taking the route of developing a new WP plugin.
Development has already started and at the moment I have a first (incomplete) version running at my development site.
Please feel free to contact me for more info (I'll be watching this posts' comments, too).