Luke Smith has posted an interview with Freeverse's Bruce Morrison over at Bungie.net to explain how and why Marathon is coming to the Xbox and the role that the Bungie community played in it:

In 2004 Freeverse hired two new employees, Mark Levin and Bruce Morrison. You might know Mark as "Count Zero" or "HaveBlue" from HBO and Bruce as "Hippieman" from Marathon.org. It wasn't long after that Mike Watson or "Patient Zero" from Bungie.org came to work at Freeverse and finally Steven Cento from the Myth community joined up in 2006 (He had done contract work for Freeverse for many years previous).

This was the core team to work on Marathon, and with good reason, everyone on the team had worked in Forge and Anvil. Mark worked on Aleph One and Steven had designed Myth tools and was familiar with Bungie's file format. It was a perfect match. These guys idolized Bungie and Marathon, and they knew how to treat the series with the respect and love it deserves.

Word is that Marathon 2: Durandal was chosen for XBLA because it is "the more accomplished and feature rich game, more modes of play and a better Campaign mode" according to Smith. That's Bungiespeak for "we still have the source code."