Um, last I looked, the amount of domain-wide configuration you could do via Apple Server’s Open Directory was really, really small. Nothing, I truly mean nothing, compared to Server 2003/XP’s Active Directory. Has that changed?
Dramatically. You can lock down just about anything in the hardware and software on client Macs. Browse Apple's OS X Server pages, you'll be amazed at what you get for $500 for 10 clients, $1,000 for unlimited.