Also keep in mind that if you already have existing projects you can create a new project with intellij and simply navigate to your existing project source folder in order for it to import the existing source. This will work when creating multi-module projects as well.
I do this all the time with existing projects that have their own
Ant build scripts as well. It works like a charm. Also, the feature that was mentioned above (dropping files into the directory) works with version control systems as well. For example, if you check out files that were added to a project from your version control system they will show up in your project automatically too.
Also note that there is no reason to restart intellij. It will recognize these files while it is running.
Frank