The Problem
-----------
I used ant-1.4.1 with jdk1.3.1-01 in a Windows XP professional environment.
After I switched jdk from 1.3.1-01 to 1.4.0, Core task <property environment 
... > failed.

The xml and the results
-----------------------

-----------------------------------------------------------------------
env.xml
-----------------------------------------------------------------------

<project name="test" default="test" basedir=".">
  <property name="os_name" value="Windows XP"/>

  <target name="test">
    <echo message=" before property environment='env'"/>
    <property environment="env"/>
    <echo message="JAVA_HOME = ${env.JAVA_HOME}"/>
    <echo message="ANT_HOME = ${env.ANT_HOME}"/>
  </target>

</project>


-----------------------------------------------------------------------
JDK1.3.1_01
-----------------------------------------------------------------------

E:\java\jrt\applications>echo %JAVA_HOME%
c:\java\bea\jdk131

E:\java\jrt\applications>echo %ANT_HOME%
c:\java\jakarta-ant-1.4.1

E:\java\jrt\applications>echo %PATH%
.;c:\java\jakarta-ant-1.4.1\bin;c:\java\bea\jdk131\bin;C:\WINNT\system32;G:\ora
c
le\ora90\bin;C:\WINNT\system32;C:\WINNT;C:\WINNT\system32\WBEM

E:\java\jrt\applications>java -version
java version "1.3.1_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_01)
Java HotSpot(TM) Client VM (build 1.3.1_01, mixed mode)

E:\java\jrt\applications>ant -version
Ant version 1.4.1 compiled on October 11 2001

E:\java\jrt\applications>ant -f env.xml
Buildfile: env.xml

test:
     [echo]  before property environment='env'
     [echo] JAVA_HOME = c:\java\bea\jdk131
     [echo] ANT_HOME = c:\java\jakarta-ant-1.4.1

BUILD SUCCESSFUL

Total time: 0 seconds
E:\java\jrt\applications>

-----------------------------------------------------------------------
JDK1.4.0
-----------------------------------------------------------------------

E:\java\jrt\applications>echo %JAVA_HOME%
c:\java\j2sdk1.4.0

E:\java\jrt\applications>echo %ANT_HOME%
c:\java\jakarta-ant-1.4.1

E:\java\jrt\applications>echo %PATH%
.;c:\java\jakarta-ant-1.4.1\bin;c:\java\j2sdk1.4.0\bin;C:\WINNT\system32;G:\ora
c
le\ora90\bin;C:\WINNT\system32;C:\WINNT;C:\WINNT\system32\WBEM

E:\java\jrt\applications>ant -version
Ant version 1.4.1 compiled on October 11 2001

E:\java\jrt\applications>java -version
java version "1.4.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0-b92)
Java HotSpot(TM) Client VM (build 1.4.0-b92, mixed mode)

E:\java\jrt\applications>ant -f env.xml
Buildfile: env.xml

test:
     [echo]  before property environment='env'

BUILD FAILED

java.lang.NullPointerException
        at 
org.apache.tools.ant.taskdefs.Property.loadEnvironment(Property.java:
253)
        at org.apache.tools.ant.taskdefs.Property.execute(Property.java:183)
        at org.apache.tools.ant.Task.perform(Task.java:217)
        at org.apache.tools.ant.Target.execute(Target.java:184)
        at org.apache.tools.ant.Target.performTasks(Target.java:202)
        at org.apache.tools.ant.Project.executeTarget(Project.java:601)
        at org.apache.tools.ant.Project.executeTargets(Project.java:560)
        at org.apache.tools.ant.Main.runBuild(Main.java:454)
        at org.apache.tools.ant.Main.start(Main.java:153)
        at org.apache.tools.ant.Main.main(Main.java:176)

Total time: 2 seconds
E:\java\jrt\applications>