Tomorrow iOS 4 is going to be released for iPhone 3G and 3GS. It is a free upgrade. I tried it on my 3G and I recommend not to uprade the OS and keep the current 3.1.3 the reason is that everything is lagging and apps crashes often. There are some feature of iOS that are good to have such unified inbox and folders, but is not worth it. Of course for 3GS is a totally different story and you'll have all the features. Careful to upgrade the OS if you think on downgrade later because even tho there is plenty of info on the Internet on how to do it, you may need to work that around and would take a while. Make some back up too.