A quick primer on the new home button functionality courtesy of iPhone OS 3.0:
A single click returns you to the last apps screen you launched an app from, but if you are in an apps screen already, it brings you back to the home screen. If you are already on the home screen, it takes you to the spotlight search screen.
Instead of assigning "Double-click the Home Button" (found in Settings > General > Home) to Search, you can instead use it for something more useful, like Phone Favorites. To get to spotlight search quickly from within an app, triple-click in this rhythm "Tap - TapTap." The first backs you out of the app, the second brings you home, the third into Phone favorites (or whatever you set).
Of course, holding the home button brings up Voice Control - arguably the best addition to the 3GS.
Tip: Stuart Pomerantz