as stated in the docs (and tried by myself), in a HasOneComplexTableField, you got the possibility to uncheck a radio button in order to make the relation as null.
unfortunately i can't reproduce this behaviour in a HasOneDataObjectManager (using vista/firefox3).
@unclecheese (maintainer): i'm still learning from my own faults, so this time i did an svn update before posting here. i think of this as a low priority issue. however, maybe it could find it's way into your bug list.