Nexus One Wallpaper Toolkit
I've looked around the web for the correct size of the wallpaper and a way to control what content is visible on a specific home screen. In fact the wallpaper is not scrolled 480 pixels when you change homescreen. After various experiments I've come up with a photoshop file where the guides are ready for you to design your own wallpaper for your nexus one. It's for for you to download. Enjoy.
BTW: After I've done all the experimenting I've seen that simple math would have resolved it earlier. The whole wallpaper is 960 pixels wide and dividing it by 8 it gives you 120 pixel. This is the exact amount the Nexus One slides the background when you change homescreen.