Oh man, this is a good find.

Enesys Software have a free (as well as a commercial version with more features) SQL Server Reporting Services data extension for SharePoint lists.

It lets you build SQLRS reports from data stored in lists (any kind of list) in your SharePoint server.

The setup is a bit manual right now - it's just an xcopy of a DLL and a few hacks to XML config files - but I can at least confirm that it works at design time.

I still believe that any hard number-crunching kind of application belongs in a "real" database rather than SharePoint, but this at least will allow some simple charting and reporting out of some of the custom lists that our power-users put together.

Check out their blog, too. It looks to be pretty new but I'm sure it'll have some good stuff on it.