I agree. Im not even sure the multitasking is gonna perform all that well with the 3gs, both cpu and battery wise.
I have the 4.0 GM on my 3GS and multitasking is performing wonderfully - I know that no App Store apps support it yet, but even using it between Apple's own apps (even those that constantly use data like Maps, Safari, etc) I've had no real issues CPU or battery-wise. Obviously this will change a bit when you get data-heavy apps (like radio apps, Skype, etc), but that's a given if it's got a constant open connection. And right now, if anything, I'm getting better battery life than I did on 3.1.3.