According to
this article on HomeLan,
Sierra has officially revealed that
Tribes: Fast Attack has been cancelled. Alex Rodberg, Tribes project manager, explained that "the main new feature in Tribes: Fast Attack (the single player campaign) was simply too similar to the single player modes in Tribes 2 and not what Sierra wanted to offer for single player PC Tribes play. Instead, Sierra plans to relaunch Tribes 2 as a retail PC product at the end of September for a bargain price of $10."
Check the link above for more details.