Andrew Meggs has posted a follow up in the MWA forum about GameSpy and how its service would affect and influence Myth III. He also goes on to and says nothing is official and explains the current status:

So, the story for Myth 3 is this: we really don't want or need our own network infrastructure. GameSpy is a possibile provider, but nothing is final and any announcements are premature. Myth 3 will have its own network interface and some very special Myth-specific networking features, regardless of who's actually worrying about what happens if a hard drive fails or a power supply goes bad. And isn't that what really matters?

Mythers across the web have expressed concern with a GameSpy service hosting Myth III's multiplayer because they question the value of community it will deliver. From the responses Mumbo Jumbo employees have been offering, it probably shouldn't be much of a concern; they're all Myth fans and want the game to be a success in their eyes as well as the consumers. They know that they have high expectations to meet, but this is the team that can do it because they truly want to.