Apple [AAPL] announced the iPhone 4 this week and again repeated the multitasking demonstration that iOS 4 has the ability to do. There's a slight problem though in that not all applications (if any), can multitask other than the official ones and the ones used for testing in the demonstrations.
Apple confirms that this is the case on the Apple website. When you look over the details of multitasking you might notice a small 1 next to the brief description. When looking up that reference it says...
"Multitasking is available with apps that have been developed to work with iOS 4."
This means that all your 3rd party apps that are running now are basically not going to be able to multitask until each of the developers update their apps to function on iPhone 4. That's not to say apps won't work on iPhone 4 or iOS 4 running on a 3GS, but to make use of multitasking in a useful way, developers will need to resubmit an updated version of their apps.
It is unclear at the moment how long it will take for all of your apps to be updated. Some will be ready on day 1 we believe due to the shear amount of updates we have seen to some apps, but others and especially the free ones, might take a while before the necessary few lines of code is added in to the application.
I personally don't think this is much to worry about and all the popular apps I expect will be done relatively quickly. It's impossible to know for sure though till it launches and we see what is ready to be updated.
Via: Pocket Lint