You sure you have a 3rd generation iPod touch? A common misconception is that 8GB models are available as both a 2nd & 3rd generation. Fact is though, it is not. Only a 2nd generation iPod touch was released, but since it was still available through the 3rd generation, people who bought one then think it is a 3rd generation, when it wasn't. Just to make note though that the 4th generation 8GB model is indeed a 4th generation model. (Apple changed strategy by then).
Getting back to the original question, the 2nd generation 8GB model does not get all the features of iOS 4.0, multi-tasking & wallpaper among them.