antcontrib.properties does not have an entry for ForTask. I observed this in version 1.0-b3 and in the current trunk code. A line similar to the following should probably be added to the file:
I also noticed this. There is a workaround:
<taskdef name="for" classname="net.sf.antcontrib.logic.ForTask">
<pathelement location="path/to/ant-contrib-1.0b3.jar" />
but it's irritating having two taskdefs for ant-contrib.