On January 8, 2013, Bethesda Softworks has registered a Fallout trademark for "Entertainment services in the nature of an on-going television program set in a post-nuclear apocalyptic world." You can view the United States Patent and Trademark Office entry here.

Interestingly, a day later, Erik Dellums tweeted that there may be more Three Dog coming.

Correlation does not imply causation, and such trademark had been registered before in 2009, so it might just a trademark registered "just in case", but... let the speculation begin!