There may be times in WordPress where you would like to load the WordPress environment manually, and prevent plugins from loading.
Two instances where this is ideal are:

When loading an inline frame with Thickbox or Colorbox.
When parsing AJAX requests in an external PHP file.

To further give justification for this technique, I had a major plugin conflict

More at: http://weblogtoolscollection.com/archives/2010/01/10/preventing-wordpress-plugins-from-loading/