Introversion has confirmed that both Darwinia and Multiwinia: Survival of the Flattest will be arriving on the Xbox Live Arcade service this Autumn 2008. The new package (containing both games) will simply be called Darwinia+, and the developers have promised to release Multiwinia as a separate title on the PC as well.

Darwinia+ confirmed for Xbox Live Arcade (Introversion)
Darwinia+ confirmed for Xbox Live this Fall (Joystiq)