Are you using 4.0.2 final? If so try dropping back to 4.0.2RC1. There appears to be issues when casting ejb3 objects with the latest release, whereas 4.0.2RC1 works ok.
I had the same problem with the 4.0.2RC1 release - i have also tried with Tomday 5.5.2 and 5.5.9 ..
I wish that was the problem :) I shall keep searching