Not every developer gets the headline treatment when they make an announcement like this, but Bungie is not just any developer. I don’t have a lot of love for Halo, but I respect Bungie and they’re a huge force in the market, so when they confirm their next project is “a massively multiplayer action game”, people like Blizzard tend to take notice.
Beyond what Bungie’s David Aldridge said at GDC today, there’s no new information, so we’ll probably have to wait until E3 before we hear anything new. It’s exciting, though — more competition for WoW is always good, in my opinion. (And here’s hoping it’s based in the Marathon universe.)