This is a known issue when exporting on Mac OS X Leopard and has been fixed in the OmniPlan 1.5 beta releases. You can download OmniPlan 1.5 RC 2 from our website here:
http://www.omnigroup.com/applications/omniplan/beta/
I'm so sorry about your data loss, I know it's not fun having to reconstruct everything from scratch. :-(