I dug up the following 2 quotes from Brad Oliver and Glenda Adams on the issue (God bless Google):
From-
http://apolyton.net/misc/interviews/ac_bradoliver2.shtml
"DanQ: In terms of multiplayer capabilities, will it be compatible to allow Mac SMACer's to go head-to-head with their Windows-based counterparts?
Brad Oliver: Very sadly, no. The Windows port uses DirectPlay, which is a proprietary networking protocol designed by Microsoft and which only runs under Windows.
It would be a near-Herculean task to reverse-engineer DirectPlay to get it to run on the Mac, Linux, etc..., and to date Microsoft hasn't expressed any interest in helping port DirectPlay to another OS. I doubt very much that Microsoft would welcome someone reverse-engineering their code without their assistance, and no one really wants to face Microsoft's legal team to find out ;-)
The only way at this point for Mac users to play with Windows users would be if Firaxis were to add networking support that didn't rely on DirectPlay (or any other proprietary scheme) to the Windows version, or Microsoft were to step up to the plate and help get DirectPlay running on the Mac. DirectPlay is very convenient for Win32 programmers as it's a nice high-level networking API, so it's far too tempting for them to ignore. The downside is that it forces a game to only be compatible with Microsoft operating systems. I dream of a day when game developers use open networking protocols.
To that end, I know of at least one currently: OpenPlay, an open-source library which uses a high level API (similar to DirectPlay) and runs on both Win32 and the Mac. It has been field-tested in Bungie's "Myth" series of games. If game developers would use it or something just as open, then it would make our lives a lot easier and would stem the complaints from users who wish to play cross-platform network games. I really can't stress this point enough. If a game company wants to make a game run on more than one platform, use a non-proprietary networking API
"
And from-
http://www.insidemacgames.com/features/view.php?ID=138&Page=2
"IMG: And theres always the issue of networking. Lets call it the biological equivalent of a mouth, where one computer has to talk to another. In the case of Macs and PCs, this can be even more difficult because they often speak different languages. Specifically, many PC games use Microsofts networking protocol DirectPlay.
Actually, DirectPlay can be thought of as a virus. Mac developers can make software that works with an existing version, but Microsoft frequently updates the code mutates the virus, if you will meaning Mac game developers would have to constantly change code in older games to keep them compatible with their Windows cousins.
Glenda: For networking, DirectPlay is a real thorn in the side of Mac gamers. Microsoft hasn't published the internal DirectPlay networking protocols, so the only way to make a Mac game compatible is to reverse engineer DirectPlay and build a compatible library on the Mac. This has been tried in the past, with varying degrees of success. The problem is even if you can get Microsoft to not sue you for reverse engineering DirectPlay, you still have to end up rewriting your Mac DirectPlay library every time Microsoft updates DirectX. So you get stuck in a never ending process of maintaining a network library where you never know what will change next. The scope of a Mac DirectPlay library is large enough I don't think its economically feasible to do unless you can use it for several games. But since every game might use a different version of DirectPlay, or different features of the networking, you may not be able to reuse the Mac DirectPlay code from one game in another.
With all the legal and technical (and financial!) hassles, nearly every publisher has opted to do Mac-to-Mac network play only for DirectPlay games."
