However, before you upgrade, here is what you should know. iOS4 was specifically created with the iPhone 4 in mind, and for older devices it is a big battery drain. Here is a tweet one of my Twitter friends posted who is using iOS4 GM on an iPod touch.
So, with multitasking and wallpaper support, your older iDevice will drain battery much faster than the iPhone 4, which has a better and newer battery itself.
Again, you can save battery life by turning off Wi-Fi and lowering the brightness of the device, however, with normal usage it will only run for 3 hours or so.
This goes on to show that other than iPhone 4, the earlier iDevices were not created for multi-tasking and iOS4 update will definitely have a huge impact on it. Of course, iOS4 brings a lot of new features, but those features come at a cost, so upgrade only if you are certain about it, because thanks to Apple, there will be no rollback.