Nothing in seam limits its use to web clients, though obviously a large amount of effort has been put into making JSF development smoother. A swing client can access a seam backend via EJBs remotely (right? I don't think you'd get conversational support though that's less important anyway with a stateful client). You can also use the webservice capabilities of JEE 1.5. Simplifying this is a major focus of current seam development in fact.
There's certainly no reason that Seam has to be limited to web application development. We've even talked some about using it in Swing applications. However, I don't think anyone sees it as a huge priority right now. I'm sure we'd welcome contributions from anyone who wants to use Seam outside of web applications.