For some reason it I was not manage changing the JRE to be used by ANT (within WSAD). I can change the JRE for my project / workbench and the build works fine outside of WSAD, however when I run ANT within WSAD it always uses the IBM JRE 1.3.1 I am using WSAD 5.1.2
I'm not sure if it helps you. In Eclipse there is plugin org.apache.ant_1.6.5 and within this folder in bin subdirectory there is ant.bat which is using JAVA_HOME env var. So maybe you should do the change here.