Passing the Hudson env var $WORKSPACE to Ant stopped working. I had my Ant build working and after upgrading, it stopped building in the workspace. After some debugging, it appears that Hudson doesn't replace $WORKSPACE with the env var value when calling Ant. I had to change my Ant script to get ${env.WORKSPACE} instead.
Previously, I had defined the variable in Hudson like this as : myvar=$WORKSPACE and this worked.

Both methods are described here : http://wiki.jenkins-ci.org/display/JENKINS/Building+a+software+project#Buildingasoftwareproject-AntScripts