iOS 5 will definitely work on 3GS. I am skeptical about the performance on outdated hardware but I don't think it will be that bad. See the bottom of this page

iOS 5 will definitely work on 3GS. I am skeptical about the performance on outdated hardware but I don't think it will be that bad. See the bottom of this page for supported devices: iPhone 3GS iPhone 4 iPod Touch (3rd and 4th Generation) iPad 1 & 2 http://www.apple.com/ios/ios5/features.html http://www.apple.com/ios/ios5/features.html