The problem was the MySQL ODBC Connector. I inadvertently installed the
latest version 3.51.15 which doesn't work. I uninstalled and backed up to
version 3.51.12. Works perfectly now on both Excel 2003 and Excel 2007.

Whew. I thought it was going to be some nasty Vista incompatibility.
Amazingly, my entire application runs perfectly on Vista.

Problem solved.