The management of several applications called multitasking is the great new feature on Firmware 4.0, but will not be possible for old models like iPhone 3G or iPod touch 2G.
Cdevwill has found a great thing !! It seems that we could activate this feature on iPhone 3G just by modifying the file N82AP.plist.

• You just have to modify this file .plist :  Multitasking=TRUE instead of FALSE.