Some applications have a check button for "start at login"...
Start on login http://b2.s3.p.quickshareit.com/files/growlstartup49525.png
How would you implement this?
This will do it, according to Justin Williams: http://carpeaqua.com/2008/03/01/adding-an-application-to-login-items-in-mac-os-x-leopard/
