I followed all of the instructions a couple of times, and I'm just not having much luck tonight. Where do I start?
advanced-cache.php is in the proper place. The config file is there next to it, and the plugin has no problems writing to it. Both .htaccess files were written by the plugin, too. wp-content and wp-content/cache are writable. wp-content/cache/meta was created, but there was no wp-content/cache/supercache. I manually created the supercache directory, but that didn't help.
I have turned the plugin off and back on, deleted the cache, expired the cache, etc. Nothing seems to be working for me. I'm testing using perl's command-line GET script (no cookies guaranteed), and I'm never seeing any of the cache comments at the bottom of the page.
PHP logging was enabled to a file, and that file never has any errors from the supercache plugin.