I thought the way in which Apple has implemented multi-tasking in OS 4.0 was quite clever actually. Theoretically speaking, it should do everything the end user wants it to do. At least, it's a 100% step up from the current OS. So people shouldn't really complain.
The notification system, however, has gone from terrible to possibly the worst system on earth.. or well, on computerized earth. Because now the multi-tasked apps will spam you with notifications, keeping you updated on their status (for example task completion), and now we've also gotten local notifications.
So you'll end up with the following scenario: you receive a text, push notification, local notification, alarm, battery 20% message, etc etc., and you will already have like forgotten that you'd received a text.
Apple's current implementation of notifications is a shame, especially when you compare it to Android, which is a much better and much more useable and organized approach.
OS4.0 seems to be a game changer for iPhone owners, but man, I wish Apple would step up on the notifications one day.
Oh and, I honestly can't comprehend the fact that some people are writhing in agony that their 3G won't support multi-tasking. I too own a 3G, and am perfectly capable of understanding why it won't support multi-tasking; the thing is already running on it's last legs on plain 3.0. There is no way in which you could implement a smooth running multi-tasking solution on an outdated platform such as the 3G.
Plus, the two year 3G contracts are due to be refreshed for the 4G anyway.