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With MySQL 5 you can share tables across databases. Also, it is very easy to setup shared core files on Linux systems using the cp -Rvl command. This is one of the simplest ways to have a multi-site/multi-domain setup.
If you have LDAP/Active Directory for user authentication, basic mutli-sites are built into the core. Things like post tracking are not shared, but authentication is.
Now, if you're talking "on the fly" creation of multi-site/multi-domain XOOPS sites, where you just upload a module and go, there is no current solution.
Many of us would LOVE to see such a thing.
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.