It’s certainly exciting that the iPhone is getting multitasking… but with only the very last generation of iDevice’s supported, a lot of people are going to be left behind. Even Apple seemed a little bit embarrassed not to be rolling multitasking out across all devices.
Presumably, the issue is one of horsepower, but not complete inability for pre-3Gs devices to multitask. A developer has discovered that by switching just one variable, you can enable multitasking on the iPhone 3G fairly easily.
Personally, I wonder if Apple wouldn’t have been wise to be a little more flexible on their rigid performance expectations for multitasking. When Apple introduced the App Store, they essentially eliminated the biggest and most obvious reason why the average iPhone owner would choose to jailbreak their devices: the ability to run third-party software. Getting multitask on older iPhones and iPod Touches is going to be a big reason for people to start jailbreaking again.