just tried IOS 4.0.1 on my 3G iphone.
its SLOW ,but at least i saw the Multitasking which is really Nice!
the steps are simply Download IOS 4.0 + IOS 4.0.1 + redsn0w win 0.9 5b5-5.zip Or Mac. [ EASY TO FIND, JUST SEARCH. ]
then Restore your iphone 3G or ipod 2G to IOS 4.0.1, then it will ask for sim card to activate it, leave it and open Redsn0w 0.9 5B5-5 And select the IOS 4.0, then continue its very simple.
waiting for my 3GS Jailbreak now .
REMEMBER : THIS ONLY WORKS FOR IPHONE 3G + IPOD TOUCH 2G .
if you want 3GS or ipod 3G, you have to wait some time.
here some screens of my 3G:-
BEFORE:-