Vinyl wall decals are another good option for spicing up your white walls. They are often colorful, customizable, large and removable. One of the downsides is that they can be pretty expensive, depending on the size and style that you desire.
The decal above came from http://www.etsy.com/shop/singlestonestudios. There are plenty of shops on etsy that carry vinyl decals. Other carriers include http://www.whatisblik.com/, which also has some pretty awesome wall graphics. Stay tuned for more decal carriers once I dig them out of my bookmarks ;)