The test file format (which is the only file I was attempting to import during these tests) is copied straight from the wiki sample and was verified by you above. Permissions on the file are 777.
I appreciate your help, but I decided to give up and enter the posts manually yesterday. This also gave me the opportunity to clean up some ugly stuff in the posts themselves, and fortunately it didn’t take that long. Probably about twice the time I had already spent trying to get the import working. :-|
I’m still curious as to what might be going on, since manually entering the posts worked in this case because of the relatively small number of posts. It wouldn’t be practical for a blog with hundreds+ of posts.