When you installed an app in iPhone OS 2.2 and earlier, it was put in the next available slot of your first home screen. This was annoying if you like to maintain a clean first screen.
In 3.x, new apps are added to the first slot on your second (or next available) screen, leaving your home screen alone no matter how many open slots it has. Oddly, when Voice Memos was introduced when I installed 3.1 on my iPod, it was added to the first screen.