This has happened to me, too, but AFAIK, it is *not* OS X that randomly switches between keyboard layouts. You do this yourself, by trying to call up Quicksilver *before* it has actually launched. (It takes a while for it to load after your system has booted.)

Your hint is cool & definitely useful; the reason why it will work even with Quicksilver installed is because Quicksilver has not launched by the time you get to the login window.

In OS X Tiger in --> System Preferences --> Accounts --> Login Options, you can also tick "Show input menu in login window", and you can choose one of your keyboard layouts from a drop-down menu in the login screen.

patrice
http://www.patriceschneider.com/apple-osx/blog/

This hint will also work if you are using a password protected screensaver. In this case, quicksilver will be already running, so it seems that when there is a password dialog on the screen, keystrokes are not passed on to any other apps.
This makes sense, because if you move the cursor away from the dialog box you get a spinning beachball.