Again, if you're getting the error and your include_path is the one specified in php.ini, then your httpd.conf file does NOT allow .htaccess files to override configuration options. (Read: AllowOverride Options)
If you'd prefer to change your include path in a PHP file like config.php, by all means give it a shot, but to do this you need to have PHP configured to allow modifying configuration parameters at runtime, which is NOT allowed in safe-mode, nor do most people allow this in non-safe-mode installations. It's risky.
Take care,
Nik
http://www.bigaction.org/