Business Week, Microsoft's general manager of global content acquisition and strategy, Ross Honey, painted a somewhat bleak picture for web content on Xbox Live. While The Guild has managed to succeed on Microsoft's platform, it has done so via a sponsorship deal with Sprint. Apparently, finding a different way to monetize a web series -- like inserting ads into programming -- is generally more trouble than it's worth.
Furthermore, Honey tells Business Week that it's important that Microsoft finds content that will "move the needle" -- in other words, content that will appeal to the Xbox audience. Honey pointed out that "gamers love watching things about gaming," noting that the World of Warcraft and Guitar Hero inspired South Park episodes are the most popular shows on Xbox Live.
Failing that, you could always make your web series about Halo.