I spent an hour fruitlessly trying to figure out what had happened, then I gave up and rebuilt it using another approach. I asked around on usenet and got a quick and accurate response
The response pointed to this Microsoft kb article:
You cannot change, add, or delete data in tables that are linked to an Excel workbook in Office Access 2003 or in Access 2002The kb article includes a set of workarounds that are pretty humorous, along the lines of 'open Excel and type in the data'. Evidently Microsoft has fallen afoul of a patent (a stupid patent I'd wager).
...This expected behavior occurs when either of the following conditions is true:
• You have installed Microsoft Office 2003 Service Pack 2 (SP2).
• You have installed the update for Access 2002 (KB904018) that is dated October 18, 2005.
Because of legal issues, Microsoft has disabled the functionality in Access 2003 and in Access 2002 that let users change the data in linked tables that point to a range in an Excel workbook. However, when you make changes directly in the Excel workbook, the changes appear in the linked table in Access.
Wow. I wonder how great a productivity hit this is around the world. I would really like to know why Microsoft didn't put a prominent warning in the installer for SP2 or on their SP2 web site. Did the courts forbid notification? Did Microsoft think no-one would notice?
This combination of non-notification and loss of functionality from a "service pack" is unprecedented. Microsoft seems to have become a very troubled company very quickly.