The next version of OS X Server, Lion OS X Server, will arrive alongside OS X Lion in July. Rather than provide two seperate downloads, you will be able to purchase OS X server as an add-on for OS X Lion for $49.99.
Feature wise, there isn’t much new in OS X Lion Server. What is of interest is the installation instructions. It would seem that you will only be able to update to OS X Lion Server if your server is already running OS X Snow Leopard Server.