Lion Server can only be added onto Lion OS. Lion OS needs Snow Leopard to be upgraded from the App Store. However, I was successful in buying Lion OS via App store on my iMac at home. Then I burned an image of the installation file and did a clean install on my MBP and Mac Mini.
After the Mac Mini was fully installed to Lion OS, I went onto the App Store and purchased and Downloaded Lion Server Add on. I am also able to download and install the Lion Server to any of my other iTunes registered Macs if I wish.
To be clear.. Lion Server is not packaged or sold as an OS like Snow Leopard Server. It is an add on that requires Lion OS as a base.
The upshot of this is you can run the Server App in the background of a Mac using Lion without needing to dedicate a single computer as a Server machine. Of course this assumes you do not need heavy server work.