Go to the Market and search for "Home button" and you'll find a number of apps like what you're looking for. I recommend "Home2 Shortcut" it allows you to re-program the buttons on the bottom of the screen to launch applications, or perform other functions. This app uses 'double taps' of the buttons to perform the programmed function, I'm not sure if it also uses the 'hold-down' method or not, but give it a look!
