According to the official wiki of Fallout, it was registered by Bethesda's parent company, and the IP can be traced to them. This proves that it is from them. However, the site is registered under GoDaddy and not CLC Hosting (the one they usually use). It is also hosted in Europe, and not in USA.
Also, if you look in the properties of the site it includes a file named Fallout.CS
And, the font used is in both Fallout 3 and Fallout New Vages.
This proves that Bethesda (or a company owned by the same parent company) may be developing a new Fallout game. But, I wouldn't get your hopes up. There is no set date, and the website was registered only a couple of days ago.