I don't think they will. I'm not sure how the newer iPhones/iPod Touches handle multitasking, but I've got a 2nd gen iPod Touch and it doesn't run very well.
Menus are laggy sometimes when swiping, and it takes a much longer time to close an application as compared to before. I jailbroke my iPod Touch using redsn0w, so it runs multitasking, but honestly I feel like removing it. Most of the time it doesn't even work anyway. Not only that, it hogs up the memory just by staying open, and it makes my games laggy, so I end up having to kill al the non-active apps using SBSettings.