However, if you have a 3G iPhone, it has been already shown that the new iOS stresses the device considerably much. I recommend you not to update yet, and wait for some videos on YouTube to see how fast it moves. Apple is always making older devices (this time is 3G's turn) to move slowly with new software, in order to make people buy new devices (3GS or 4). And if you're on 3G, you won't have multitasking and wallpaper image (so there is no real need to update, in my opinion).
What else you need to know is that if you ordered a new 3GS today, it is already being delivered with iOS 4 so if you need to unlock it, there is no jailbreaking solution yet. Don't understand me wrong, iOS 4 has already been jailbroken, but the 3GS and the 4 will surely come with a new boot rom installed so you can't jailbreak them yet.
Anyway, if you need a jailbroken device, do not update yet, as the situation is not clear how to jailbreak iPhone 3G/3GS/4.