In the latest release of SharePoint Patterns and Practices ( Developing SharePoint Applications) the P&P documentation includes a section comparing and the use of SharePoint lists to database tables. It is a short read and worth the time to read since this topic surfaces often in discussions.
While SharePoint lists provides a quick and easy way to store data it does has some limitations and should not be used where a true relational data store is required.
Using SharePoint Lists vs. Database Tables