This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
GQuick does discuss what Grails is best for, which (in my somewhat biased opinion) is web applications, large and small, that target the Java Virtual Machine. Grails produces a Java .war file which can be deployed to any standard Java web container (Tomcat, Jetty, JBoss, Glassfish, WebLogic, etc.)
I don't cover using Grails for desktop development, however you could conceivably use Grails to build the server-side component of a distributed application that included a desktop component. As an example, at the recent SpringOne 2GX conference in New Orleans, Jeff Brown built a desktop application in Griffon (the Groovy Swing MVC framework) that accessed a Grails application via a webservice.