oops, better get that fixed ;)
I believe this is a stateful session bean bug in the underlying EJB3 implementation (EJB3 RC3). It should be fixed in the new version of Seam/EJB3, which will come out very soon.
In the meanwhile, you can just wait for a few minutes and reload the http://seam.demo.jboss.com/home.jsf page and the error would go away. Or, you can download the example and run it on your own machine.
I was just letting everyone know, as this wasn't always the case.
I've got it downloaded, but was just doing a quick reference lookup on the web.