The latest iOS4 update for iPhone 3G does not enable multitasking and wallpaper support due to Apple restrictions, however, the iPhone jailbreak tools sn0wbreeze 1.6 and redsn0w 0.9.5 beta will allow users to enable Multitasking and wallpaper support for the home screen while jailbreaking the iPhone.
Screenshots for the new updates were shared by the iPhone dev team and the sn0wbreeze developer. The screenshots show options for users to enable multitasking, wallpaper support and battery percentage on iPhone 3G.
So rest assured, once these tools are out you will be able to enjoy both multitasking and wallpaper for the home screen on the iPhone 3G. The wait for these jailbreaking tools for iPhone for iOS4 should not be long now.