I figured it out...pretty interesting.
I had just installed Quartz (a scheduler service) as a SAR in the jboss deploy directory. The SAR references my EAR since it needs to reference once of the classes. For some reason, this makes the EJBs unaccesible via RMI. A strange classloading issue I suppose...
I am having similar problems with local session chaining. Just by coincidence, we are also using Quartz, v. 1.3.3.
I'm not sure what's going on.