relate problem could be found at http://forums.bea.com/thread.jspa?threadid=200011896
when we build spring 2.5.5 at eclipse platform(weblogic workshop), i meet the same question.
it is becasue workshop use ant 1.6.5
please select windows=>preferences=>runtime
on classpath tab, select ant home entries
add external jars as below:
d:\spring_work\spring-framework-2.5.5\lib\ant\
ant-junit.jar
ant-launcher.jar
ant-trax.jar
ant.jar
and up these jars before the original ant runtime jars.
it means you are using ant-version: apache ant 1.7.0 instead of ant 1.6.5.
now
task support the "base" attribute.