i found a solution for this problem. it is described in:
http://www.csd.abdn.ac.uk/~bscharla/teaching/CS5302/practicals/practical9.shtml (see "section" setup)
http://www.dcl.hpi.uni-potsdam.de/research/asg/ws4ejb (see "section" xdoclet integration)
regarding to this two sources, following worked for me:
i downloaded the latest xdoclet-jars from sourceforge.net (version 1.2.3) and unzipped it into %eclipse_home%/plugins/org.jboss.ide.eclipse.xdoclet.core_1.4.1
after that you have to delete the old-jars (possible 1.2.1 or anything else) and then it was possible to change the ejbspec to 2.1.
but i got another problem:
jbosside doesn't support the generation of the file "webservices.xml" neither in the 1.4.1 nor in the 1.5m1 version. i guess you have to change the existing doclet- and build-files to let eclipse generate the necessary files. maybe somebody has already a working solution for generating the files.