Now: Five different wikis have implemented the new look as part of the public beta. Viewing these wikis with the new look can be done in your preferences (Special:Preferences) on the participating wikis; it will not affect the function or appearance of other wikis (including The Sims Wiki).
October 6: The new look becomes available to all users on The Sims Wiki or any other wiki, and all newly-created wikis after that date will feature the new style exclusively.
October 20: Everyone will see the new look by default on all wikis, but will have the option to go back to Monaco (the current skin) under Special:Preferences.
November 3: The option to use Monaco is disabled on all wikis.
The new skin is going to have a massive impact on the appearance, function and layout of The Sims Wiki, and it may take many weeks or months for the wiki to return to full functionality after the change becomes permanent. There are ways you can help, though. You can participate in the Skin Project, you can report issues with pages under the new style, and you can offer feedback about how The Sims Wiki implements the new skin. Your help can make this transition occur more quickly and more easily, and will allow The Sims Wiki to continue to provide as much information as we can about The Sims.
More information can be found at Community Central, and specifically in these blog postings: