I tested this against rc2 and current trunk.
If you call <ivy:retrieve> and one of the modules to retireve contains custom attributes within the default namespace ivy fails, although the ivysetting have the validate flag set to false.
Download the zip file an extract the files. Put an ivy jar in the same directory or change the path withing build.xml.
run 'ant publish'. Afterwards run 'ant retrieve'. The retrieve fails.