the scenario:

I create a team site
I create a new Calendar and add some events
I create a new Document library and add a lookup field to the Title column from Calendar
When I choose new document in the Document Library the document opens in Microsoft Office 2007 and shows the document information panel
The lookup field to the Calendar shows no items
If I change the document properties in SharePoint the Calendar lookup field shows all events.
In a different scenario I created the Calendar AFTER the Document Library and then the document information panel worked fine.