Tho not sure, I would say no, you will not be able to play it with PC users. I'm only saying this due to my past experience with online Mac games and there is a large chance im wrong.
I wish i saw this before i picked up AOE3. Isnt Civ a turn based game though? How well does that end up working out with a 4 on 4 or whatever? Do people get irritated with the waiting?
I emailed Macsoft in January no my disappointing purchase once I found out my girlfriend who has a Dell and I who have a MBP could not play. There response to my question (when can I play her) was "I will pass your concern on to the higher ups. At the time there is no Mac/PC networking and there has not been any development. Thank you"