For those who owns iPhone 3G or 3GS, you will be able to upgrade your iPhone to the latest IOS 4 that supports multi-tasking as one of the new features. Unfortunately, multi-tasking is still not supported in iPhone 3G even if you update it to IOS 4.
To upgrade, you just need to plugin your iPhone 3G or 3GS to iTunes and it will detect the new IOS 4. Remember to update your iTunes to the latest version, 9.2.
PS: so who wants to sell me your 3g iphone? i need a door stopper