The move is part of the deal that Microsoft made with multiple streaming providers to allow their content to be streamed to the Xbox 360 platform. Comcast and HBO are two of the content providers that are part of this deal. Other providers are said to be on board, but Microsoft has not yet announced who they are.
The Verizon FiOS streaming using the Xbox 360 is different from the AT&T U Verse support for the Xbox 360, where the console is actually used as a cable box with the Xbox 360 AV Remote. Additional streaming support with other cable companies is said to be coming in the months ahead, but it is not clear yet which cable companies will be next to offer it.