Thanks Curtis, for solving a long standing mystery for me.  Lotus Notes maintains a "Keywords" property for every document (one of several attempts by Lotus to provide "knowledge management" features in the Notes platform).

It turns out that this data is stored as a simple item in the Notes document named "$$Categories".  This means we can migrate it!  To extract it using Notes Migrator for SharePoint, simple create an Item column in your Source Data Definition that reads from the "$$Categories" item.  You can then map that column to anything you want in SharePoint 

Here is a 2 minute video from Quest sales consultant Curtis Kelley that shows how this is done:  http://screencast.com/t/poNWVn37Aj