The answer will be false out of the box because the remoting transport is completely different. The only way this would be possible is for the ejb2.1 deployments to use a new detached invoker that is based on the same remoting implementation that ejb3 uses. This should be possible.
Actually, I think Scott is incorrect, but maybe he understood the question better...
EJB 3.0 has adaptor APIs for EJB 2.1. Basically you can write an EJB 3.0 bean and there is an adaptor pattern for Home interfaces and Remote interfaces.
So, yes, it is possible...
Here's more information:
Its better to download the EJB3 distribution and play with it yourself as this doco and tutorial is in that zip.
I viewed his question as could he taking an existing ejb-ref that points to a remote jboss-4.0.x/ejb2.1 implementation, and simply point that to a remote ejb3 implementation, and visa-versa. The forward direction would probably just work if the ejb3 client jars were included, but exposing a legacy jboss-4.0.x/ejb2.1 endpoint to a remote ejb3 component might not. Its just a question of how well the legacy and new remoting transports integrate. Its something that needs to be tried.