A much-needed feature that finally made it into Ant 1.6 is described on Stefan Bodewig’s weblog:

With the new launcher code, the directory .ant/lib in your home directory is treated just the same way as $ANT_HOME/lib by Ant. This means that all jars puts into that directory will be available to all Ant tasks or can contain Ant libraries. No need to fiddle with the Ant installation in a multi-user environment.