Customizing Maemo
This article will someday describe how everybody can customize Maemo5 to make it as personal as possible. There is a discussion concerning customization in http://talk.maemo.org/showthread.php?t=32845
Wallpapers
There is allready http://www.n900wallpapers.com/ page for wallpapers. Unfortunately you need a .desktop-file if you want to configure panoramic wallpapers. But situation seems to develop quickly.