How well? Almost any UNIX based computer can replace quite a few Windows based servers. Windows admins have learned not to load up each server with to many functions or you get stability problems. This rule of thumb does not apply to Solaris, Linux, BSD or Mac OSX. In a server situation OSX is just another UNIX" because (1)you don't use the graphic interface in a lights-out server room. All the server functions, apache, samba, bind and so on are the same programs wetter Solaris or OSX.
Mac OS X has an Active Direcoty plugin which enables the Mac to be bound to an AD master for user policies. Fire up /Applications/Utilities/Directory Access and double click AD.
The SMB client is so so... Sometimes a Win2K3 server has to be altered a bit for SMB passwords from Tiger to work...The Digitally sign communications (always) has to be set to Disabled.
If you have an INtel Mac, just get Parallels, install Windows and let your XP be connected to AD, nice and easy. 😀