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 <rmic> task support the "base" attribute.