Quirk with importing Outlook PST files into Exchange 2007

I just ran into a quirk with the “import-mailbox” Powershell cmdlet from Ex2007 SP1.
I found that if I was trying to import an existing PST created using Outlook 2003 to an Exchange 2007 mailbox, it claimed that everything worked just fine, but none of the content appeared in the mailbox.
The fix is to first do [...]