Ryan,
I realized that after I posted the above, added the name field, and get the same error one number higher (25 fields, 26 records.)
??
Also, I've tried eliminating various combinations of columns to see if one in particular was causing a problem. I got the import to work with the Name and LeadSource columns only in the table. A word to the wise, however, once a record exists in the Method database, it doesn't like it when you try to import an expanded list with duplicate records contained. I had to go in and delete all the records I imported before trying again. There needs to be some type of "what to do with duplicates" or "check for duplicates and ask me what to do" or "import duplicate records yes/no" type of functionality built in to the API tool.
This is only an issue when importing a large list. Best work around is to test import with only one or two records until you're sure it's working properly.
What should I try next?