WordPress MU

Has anyone successful defined a custom plugin or mu plugin directory in the wp-config file? I'm trying to set up a custom directory to use svn:externals to pull in all our managed plugins. But, wpmu doesn't seem to be recognizing the following line in the wp-config file.

define( 'WPMU_PLUGIN_DIR', '../mu-plugins' );

I'm trying to follow the recommendations from this blog:
http://protocol7.com/archives/2007/12/26/decent-wp-setup-2/

If this is still an issue, why not try using (oh heck, what do you call it) the actual file directory instead of a redirect?

ie: define( 'WPMU_PLUGIN_DIR', '/home/username/public_html/wp-content/mu-plugins' );