In the sole announcement on the Mytheon forums, the team posted a note that the game would be spinning down starting this past July 27th in preparation for a final shutdown at an undisclosed date in the near future. Players can no longer register for the game nor download it. Mytheon planned to launch in 2010 but was kept in beta testing far past the projected release date.
This announcement ends on a bittersweet note: "Finally, we'd like to send out a huge thank you to all our players who've joined us on this journey. Keep challenging fate, The Mytheon Team."
[Thanks to Paul for the tip!]