Warning / Disclaimer:
This trick relies on what is probably a bug to work, and may cause problems depending on what target applications are doing at the time method is applied. The system will even provide a warning to that effect when you try to implement this hint. Proceed at your own risk...
If you have ever been in a situation where you had an application running which you did not wish to continually have to "Tab" past in the ⌘-tab application switcher, it can be removed by simply renaming it while it is running, then restarting the Dock (see below for more on how to do that). The final (important) step is to immediately change the program's name back to what it originally was.
The program will no longer appear in the application switcher nor the Dock. If the application is a permanent item in the Dock, its icon will remain, but the triangle indicating active applications will be absent. Clicking the Docked application's icon will activate the app normally, but the contextual menu will be empty. Interestingly, windows belonging to the application still appear in Exposé.
Notes:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050819232526710