No, I've textured instances many times before, so this was driving me crazy. The only solution I have found is to make sure I texture BEFORE I instance the object. Then, the instanced object will have the texture as well.
The purpose of instancing an object is mainly to duplicate like objects while minimizing memory/model size. The downside of instanced objects is you can't do any editing to them such as scaling, individually...which makes sense given their purpose.

If you make a cube, then instance it...you can easily texture the instanced cube. The only reason I can find why I can't texture my instanced objects is because I duped them in the -1 X. So, as I said, I will texture them first before I dup them....kind of strange.