The buckminster.output.root and buckminster.temp.root paths get double slashes since the ${WORKSPACE} ends with a slash. Here's a sample printout:

Commandline:
/shared/common/ibm-java2-ppc-50/bin/java -Dbuckminster.output.root=/opt/users/hudsonbuild/.hudson/jobs/buckminster.test/workspace//buckminster.output -Dbuckminster.temp.root=/opt/users/hudsonbuild/.hudson/jobs/buckminster.test/workspace//buckminster.temp ...