Office 2007 introduces a new save behavior for ActiveX Controls (OCX) saved directly in Word, Excel, and PowerPoint documents.  As part of the new Open XML file format, OCX controls can be persisted as XML data if the control supports saving by IPersistPropertyBag using data types easily converted to text formats.  This new behavior offers greater compatibility with the Open XML file format, but can cause backward compatibility problems for certain controls that only know how to save in binary fashion. 

Click the link below to read about this issue and what you (as an OCX developer) should consider when implementing IPersistPropertyBag:

Changes in OCX Save Behavior for Office 2007 Documents