While undeploying the code from Salesforce org, the resultant objects and fields gets moved to Deleted objects section which is kind of soft delete of these objects. So here the problem is : if there exist any child relationships between these objects, then they must be hard deleted else if one tries to rebuild his/her org from Ant build tool, it wont allow you to deploy it saying the child relationships between XXX objects still exist.

Posted @ : http://jatin17jain.wordpress.com/2011/04/16/clean-salesforces-deleted-objectsfields/