Customizing Maemo
This article will someday describe how everybody can customize Maemo 5 to make it as personal as possible. Discussion on t.m.o concerning customization in this thread.
Wallpapers
There is already n900wallpapers.com for wallpapers. Unfortunately, you need a .desktop-file if you want to configure panoramic wallpapers although the situation seems to develop quickly. There is a conversation going in this thread, page 12 (default settings)
Themes
Available themes can be installed straight from Application Manager. If you want to make your own theme, see below.
There is an application for making themes to Maemo OS2008. This will be updated as soon as possible to cover also N900 and Maemo5. See article Maemo Theme Maker HOWTO
Welcome screen
It is possible to edit hildon-welcome. A howto is available here. Examples: "Open source. Open heart. Open future.", "The Power of Open."