The following is an overly simple example, and source:
focus the browser on the correct element (click on it, or tab to it) up, up, down, down, left, right, left, right, b, a if you've typed the code in correctly, you should discover the Easter egg. :) NOTE: like the original, you've got to be quick or else the code won't work.
And this example is more how I envisioned this guy getting used - in some random web app, as a private joke for the developers and a few insiders. Clearly this second sample is fake, but I think it gets the point across.
Postscript: (01/12/08) - if the sample doesn't work for you, from the guy who wrote the Scriptaculous sound engine:
If you have a Mac, you're all set. QuickTime takes care of all browsers. If you have IE, you're also set. bgsound is a wonderful thing - wav w/ no plug-in. Opera? Terrific. Wav files with no plug-in. FF on the PC? Only if you have QT installed. Otherwise you hear nothing. For now.