It would be extremely useful if we could add Decals in the editor and actually be able to see them. This would provide an easier way to accurately place decals without a lot of rebuilding of the world and would make adding those extra effects far easier.

As I know it's already possible to do.

http://img341.imageshack.us/img341/4375/20276328.jpg

yes, and there is sample Decal object in the SDK data for you to see