Hello. Yes Core Audio and Audio Units are two separate things. Think of Core Audio as input/output and Audio Units as plug ins. Sort of the way ASIO lets you open a piece of software on any compatible hardware and access its input and output (Core Audio), and VST is a plug in format (Audio Units). For more info check this Apple page.
Apple info
Core Audio drivers are scheduled to be released by the end of March. This will allow you to open any compatible software with your Digidesign hardware, as well as being the main sound device on your Control Panel (CD's, web sounds, etc.). Audio Units is a plug in format. There is much debate, rumor and uncertainty about this. Digidesign has not released an official word on it. It appears from what little info we can gather that Digidesign will not support Audio Units. It seems that they are banking on Rewire2, TDM, HTDM and RTAS. This will all play by the end of the year I guess.