I’ve had the same question come up a few times recently so thought I'd blog about it as i assumed this is a FAQ.
When migrating from Exchange 2003 to 2007 will Outlook need to regenerate the .OST file?
The short answer is…no, it doesn't need to be created, the move is transparent as far as the user is concerned.
The long-ish answer is…when you use the move-mailbox command the mailbox GUID stays the same across the migration. The Outlook profile is tied to the mailbox GUID, so since the GUID moves with the migration the Outlook profile does not need to be recreated nor does the .OST associated with the profile!
Written by Daniel Kenyon-Smith