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As noted here:
http://drupal.org/node/1194898#comment-6853296
The code has an error, and the above link is the fix. The next release of this code needs to have the following code added to the imagecache_create_url function... otherwise, the image is not created:
if (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC) == FILE_DOWNLOADS_PUBLIC && variable_get('clean_url', '0')){
imagecache_generate_image($presetname, $filepath);
}