OK Setting it 'null' allows the zmprov command to execute, but then not actually work. It sets it as '-1' and looking from the logs - borks.
2008-11-03 13:16:39,782 INFO [ScheduledTask-1] [name=scottr@domain;mid=1;ds=OLD MAIL;] datasource - Importing data.
2008-11-03 13:16:39,812 WARN [ScheduledTask-1] [name=scottr@domain;mid=1;ds=OLD MAIL;] datasource - Scheduled DataSource import failed.
com.zimbra.cs.mailbox.MailServiceException$NoSuchI temException: no such folder id: -1
ExceptionId:ScheduledTask-1:1225710999810:48a387c303d21a76
Code:mail.NO_SUCH_FOLDER Arg

itemId, IID, "-1")
at com.zimbra.cs.mailbox.MailServiceException.NO_SUCH _FOLDER(MailServiceException.java:180)
at com.zimbra.cs.mailbox.MailItem.noSuchItem(MailItem .java:1178)
at com.zimbra.cs.mailbox.Mailbox.getItemById(Mailbox. java:1962)
at com.zimbra.cs.mailbox.Mailbox.getItemById(Mailbox. java:1946)
at com.zimbra.cs.mailbox.Mailbox.getFolderById(Mailbo x.java:2746)
at com.zimbra.cs.datasource.ImapImport.importData(Ima pImport.java:165)
at com.zimbra.cs.datasource.DataSourceManager.importD ata(DataSourceManager.java:154)
at com.zimbra.cs.datasource.DataSourceManager.importD ata(DataSourceManager.java:119)
at com.zimbra.cs.datasource.DataSourceTask.call(DataS ourceTask.java:78)
at com.zimbra.cs.datasource.DataSourceTask.call(DataS ourceTask.java:30)
at com.zimbra.common.util.TaskScheduler$TaskRunner.ca ll(TaskScheduler.java:98)
at java.util.concurrent.FutureTask$Sync.innerRun(Futu reTask.java:269)
at java.util.concurrent.FutureTask.run(FutureTask.jav a:123)
at java.util.concurrent.ScheduledThreadPoolExecutor$S cheduledFutureTask.access$301(ScheduledThreadPoolE xecutor.java:65)
at java.util.concurrent.ScheduledThreadPoolExecutor$S cheduledFutureTask.run(ScheduledThreadPoolExecutor .java:168)
at java.util.concurrent.ThreadPoolExecutor$Worker.run Task(ThreadPoolExecutor.java:650)
at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:675)
at java.lang.Thread.run(Thread.java:595)