I recently had a problem with Grails when I started a new project in Eclipse with Grails 2.0.0. I tried adding an external JAR file to my "lib" directory by dragging the JAR file over to the directory.
In Grails 1.3.7, the JAR file would then get added to the list of grails dependences (after refreshing the dependencies). However, that stopped working. After reading some user group posts, here's what I did to fix it.
1) run "grails clean"
2) drag the jar file to your "lib" directory.
3) right-click on the project and choose Groovy Tools -> Refresh Dependencies