The information Apple is posting seems contradictory to me. They state the following:
"Now you can quickly and easily turn just about any Mac into a powerful server thats perfect for home offices, businesses, schools, and hobbyists alike. Lion Server is coming to the Mac App Store in July for $49.99."
If you click on the "How to Buy" button it talks about upgrading an existing Snow Leopard Server by purchasing the upgrade. It does not state that you must have SL Server installed for the upgrade to run. It seems to me that two or more people worded the information on the Lion Server pages.
Arstechnica has a story that says it is just the $49 upgrade:
http://arstechnica.com/apple/news/2011/06/lion-server-will-be-an-extra-50-via-the-mac-app-store.ars
We will know for sure next month, but it appears to me that Apple is selling Lion Server for just a $49 upgrade to SL Client. The most promising part to me is the hobbyists mentioned in the quote from Apple above. Of course I could be wrong in my assumptions.