Ant 1.8 significantly changed behavior of org.apache.tools.ant.Project

public void setProperty(java.lang.String name,
                        java.lang.String value)

    Set a property. Any existing property of the same name is overwritten, unless it is a user property.

    Parameters:
        name - The name of property to set. Must not be null.
        value - The new value of the property. Must not be null.


The new implementation prohibits overrides of any property. 

The new Ant implementation breaks functionality of <nb-overrideproperty> task that is critical part of Mobility project build system.

Following test returns "b" on Ant 1.7 and "a" on Ant 1.8:
<project name="test" default="test">
  <target name="test">
    <taskdef resource="org/netbeans/mobility/antext/defs.properties" classpath="org-netbeans-modules-mobility-antext.jar"/>
    <property name="test" value="a"/>
    <nb-overrideproperty name="test" value="b"/>
    <echo>${test}</echo>
  </target>
</project>