I have a Mac Mini with SLS installed, but don't use any of the Server services. Can I do an in-place upgrade to Lion Desktop when it comes out? Thanks
Short answer seems to be: No, not to Desktop.
But if you have any more recent evidence to the contrary, please comment here to let others know.
According to an admittedly old (Lion was still in pre-release) blog article, An Informative Look at Lion Server, an in-place upgrade of Snow Leopard Server will try to verify that you have purchased and downloaded the Lion Server.app first:
So this means shelling out a total of $80 for Lion plus the separate Server.app download.
There are various online guides to creating your own Lion installation USB keys or DVDs (e.g. Mashable's Lion Clean Install Guide); if you do settle for a clean install of Lion instead of an in-place upgrade, you could put just Lion on your machine, and avoid the cost of the additional Server.app. That could always be added later if you change your mind.