The same thing that I ask to my customer. i don't know how, but I found it deleted and crontab send me a lot of email with the classic message "Ez Publish get abrutly ended....", so comparing ezuser table with ezcontentobject and ez collaborationitem tables i found that an item in ezuser table was missed.
As I know there are many cases which can result in this kind of fatal error. For example, deleting parent node(seemly not the case of deleting a pending user), or deleting an object which is in workflow pending after editing (it might happen here, but that's not for user registration).
The right solution should be not permitting user to delete it when there is object in workflow pending.
Related issue: http://issues.ez.no/IssueView.php?Id=6380
Feel free to have comment on that issue if you find something useful :).
