Okay, it looks like my error was in believing that JBoss 5.1.0 was EJB 3.0 compliant. I added the jar file and application.xml markup per Wolf-Dieter's suggestion, but then I got an IllegalStateException stating "Null beannMetaData".
I tried doing more things with annotations, but the error never went away. So I eventually gave up and wrote an ejb-jar.xml file. That appears to have satisfied JBoss' need for metadata, but now I'm getting all kinds of warnings about EJB spec violations. because my bean doesn't define the require ejbCreate method, the methods defined in my interface don't throw java.rmi.RemoteException, etc.
So either the inclusion of the ejb-jar.xml has caused JBoss to treat my bean as an EJB 2.0 bean, or there's some configuration tweak I don't know about to tell JBoss that I'd like to use the 3.0 spec and annotations, rather than implementing EJB interfaces on my business beans, etc.
Any help there?