According to Hardmac, Mac OS X Lion server will come as a paid-for downloadable app from the Mac App Store



From TUAW:

The help window states, "If this Mac isn't a server, you can make it one by installing the server app." It then goes on to detail the process of installing the app and creating an administrator account. The help menu ends with, "The Server app downloads the Server Essentials software package, installs it, and configures this Mac as a server."

  Mac OS X Lion server to come as a paid for downloadable app?