The important point is that a project must have consistent naming rules across the development team. It's good practice to have a naming standard for databases, similar to the way you have a coding guidelines. If you are not working a greenfield project, which tends to a lot of the time, it's best to follow the naming rules already in use for the particular project.

With that out of the way, here are my own preferences, largely influenced by coding standards that I use (Zend Framework's coding standards).