New evidence to support this claim has recently been revealed via the U.S. Patent & Trademark Office's website. A search on the website's Trademark Applications and Registrations Retrieval (TARR) system shows Ferrari applied for a trademark of the words 'Ferrari GT California' last December, as reported by our friends over at World Car Fans. A closer reading of the search results shows that although the application has been made, it has also been preliminarily refused trademark protection. The USPTO apparently wants more information before it makes a final decision.
It is possible that Ford's Mustang GT/CS (California Special) is part of the hold-up. Such similar naming in the same product market is a reasonable cause for initial refusal of a trademark. However, once further information is gathered by the USPTO, it is also likely that Ferrari's trademark will be approved.
It's unlikely Ferrari would invest the time and money in the trademark unless they had good reason to, and at any rate, the Detroit auto show is coming in January, so we'll know for sure in a few months. Until then, all we can do is speculate, but it sure is fun, isn't it?