Maxis announced that Darkspore is for PC users only. There will not be a Mac version at the release date:

Darkspore is a PC-only game at launch. No other platforms have been announced.

So there's the "official" line. The unofficial line is very simple: we love the Mac! Many of us are Mac users! There are several Macs in the studio! Yay Mac! But ... we have only announced on the PC.