I've installed and configured the plugin with no problems on the latest version of WordPress running on PHP5. I did the little hack for PHP5 in one of the config files, and I also have made sure the WP_CACHE bit is in the wp-config.php file.
It all seems to be right, but no pages are being cached. I can verify this by seeing that nothing's being created in the wp-content/cache directory, and the WP-Cache text is not being put at the bottom of my pages. Plus the plugin options show 0 pages.
So the last thing I'm thinking is permissions, but I think those should be right too. I have all directories (including wp-content and cache) owned by the web user, and the permissions are 755 like they should be. I simply don't see any reason why this isn't working.
I've seen the other threads, by the way, and those were always either issues with the PHP5 tweak, or a problem with permissions; nobody's addressed this specific issue. If you have any ideas I'm very eager to hear them.