If you have an iPhone 3G, don’t bother with the iOS 4 upgrade as it pretty much slows down all apps/games. Multitasking can be enabled if you jailbreak it, but the feature itself is not worth the overall device slowdown. For jailbroken iPhones the Backgrounder program in Cydia does a much better job, and also works with any app. Version 3.1.3 plus spotlight disable app in cydia has yielded the best performance for me.
For those that have already upgraded:
http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3g[s]-from-ios-4-to-ios-313
