Method I found to Uninstall Lion Server
One workaround to uninstall Server is as follows (use at own risk, but it worked for me (and it may not be as good as a clean install but I see no traces of Server other than the Server.app left))...
Go to Macintosh HD > System > Library > CoreServices.
There you will see both 'ServerVersion.plist' and 'SystemVersion.plist.' If you move the 'ServerVersion.plist' to the trash (you have to authenticate it) [make sure to leave SystemVersion.plist alone], and then restart… the System Version in the 'About this Mac' will switch from 'Mac OS X Server Lion' (before restart) to simply say 'Mac OS X Lion.'
This is important because if you try to reinstall via the Mac OS X Lion Installer from the MAS, then it will reinstall either the server version or the consumer version on the reinstall based off of what version it thinks you have.
Thus, after the restart, launch the Mac OS X Lion installer again and reinstall on the Macintosh HD. This install will remove the server options and give you a brand new system install while keeping your User folders.
Again, it worked brilliantly for myself, but your milage may vary.