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.
Erik has later confirmed that he been given permission to release that tease. NMA has already supposed that this could be the start of a viral campaign.
Correlation does not imply causation, but... let the speculation begin! Stay tuned for more news as we acquire it, and be sure to visit The Vault, a comprehensive and player-maintained wiki for everything to do with Fallout! There are over 15,000 articles, so you're sure to find the information you need.