Microsoft Windows Vista
I use the dvorak layout as my primary keyboard layout. When the computer first boots up and loads vista, the standard u.s. layout is loaded, and there is no way to select the dvorak from the login screen. I have to log in with u.s. keys.
If I have logged in, and either locked the machine, or logged off, the 'choose layout' button may be present. Ideally that button would show always (since others use the same workstation).