I assume that you all were as surprised as I have been when Apple today finally (after 4 months or so of being told on a regular base) acknowledged that they trashed the 3G devices with iOS 4. It might have had to do with the fact that they have surely done a significant amount of warranty 3G replacements and it might also be related to iOS 4.1 being said to solve it, nobody will ever know.
What I know is that I lost confidence in Apple and my last nerve after my iPhone shut down after 20 hours of idle although it was fully loaded before, where it with iOS3 lasted up to a week. The only way to squeeze out more was to disable notifications, which is more than just suboptimal.
As Apple does not want to offer any solution to the problem, I went the same way a few others went and used the one solution that solves it forcefully.
As my 3G is from 2008, my warranty ran out a long time anyway, making the decision if I should „redsn0w“ the problem pretty trivial.
And guess what: The redsn0w jailbroken iOS4 installation (disabled multitasking & wallpaper features) now has the good old battery runtime and much better performance again and runs in circles around the previous iOS4 installation in performance and battery life and that even with enabled notifications.
Subjectively and from Free Memory / AppBox pro data, it seems like I even have more ram free than on iOS 3.1.3
As I always avoided and was against jailbreaking (I know that some cool tools exist over there, but to me it was a thing that is more a problem to me as iphone dev than a help but in this case I just had no options any longer), it took me a while to find some usefull article on how to do it but finally I found http://www.iphoneheat.com/2010/07/how-to-jailbreak-iphone-3g-ios-4-0-1-firmware-redsn0w/
If you download the 4.0.0 ipsw from there it will go through the installation without any problem and work fine afterwards.
I hope quite some of you will get back a usable iPhone 3G on iOS4 this way too.