We have spent a considerable amount of time getting the BDC properly configured to pull in data to MOSS user profiles from a SQL table. Hopefully this post will save others some time!
1. Make sure your app definition xml file is correct. Test it and confirm it the minute it is uploaded. Do not make frequent changes to this file and govern the changes to it carefully.
2. Search service accounts need proper permissions in addition to other service accounts
3. The SSP service account also needs permissions to the backend SQL table you are accessing
4. And finally make sure your field types are consistent across the board from the SQL table to the BDC definition to your MOSS profiles.