I Downloaded the enterprise development kit (j2sdkee1.3.1) and set the J2EE_HOME environment variable to point to the install directory. The build now produces an ear file. (I don't know if it works yet but that's another question.)
I found this one very, very helpfull because I wasn't aware that I need a J2EE distribution in addition to JBOSS. This hind should go into the startup docu. Thanx.