There will be a limited number of Special Edition Mac copies signed by Sid Meier. These are said to be "extremely rare" and will only be sold through GameAgent, Aspyr's boxed and digital storefront.
Going by the pre-order page on GameAgent, the Mac version of Civilization V will indeed utilize Steamworks for DRM. It is unclear, however, if it will make use of Steam Play or Steam Cloud. The Steam version of Civilization IV supports Steam Play so it would follow that Civilization V would also utilize the dual-ownership feature, especially since the game uses Steamworks. Shacknews has contacted Aspyr for more information about the features it will support.
Additionally, Aspyr is celebrating the announcement by putting the Mac versions of Civilization III and all of the flavors of Civilization IV--including Colonization--on sale at a 50% discount. These discounts appear to be GameAgent specific for the time being.
i, for one, have fingers crossed for steamplay
Yeah, but I wouldn't make any assumptions yet. It seems like they had to jump through some hoops to get an agreement on that for Civ IV.