Ant is using wrong java version
Just had this issue, it happened because I'd first added the build file to the ant-view when the default JRE was 1.6.
There was no project-specific JRE and I changed the default to 1.5, even eclipse was running in 1.5, and JAVA_HOME was 1.5 too. Running the ant target from the command line used JRE 1.5, but within eclipse it still used 1.6.
I had to right-click the ant target, select Run As... and change the JRE under the JRE tab. This setting is remembered for subsequent runs.
According to the ant manual, setting JAVA_HOME should work - are you sure the changed setting is visible to ant?
Alternatively, you could use the JAVACMD variable.