Someone else on another board mentioned this...
He felt Apple will hold back multitasking until they get their developers to support the Push Notification System. Once that happens, then they may slowly allow 3rd party applications to run in the background.
Whether the application is open or not, you'll still be able to receive notifications. Why is this a big deal? Imagine you "subscribe" to about ten different services that you need to stay up-to-date on... without PNS, you'll need to have all those services open and running in the background. Apple's PNS will offer a much more elegant method of handling notifications... you can "subscribe" to 100 different services and not need to have a single application running in the background and you'll still receive message and updates.
Now, once you have the PNS service available for developers to use and they all get used to it, you can start to allow 3rd party applications to run in the background that truly need to continue processing while in the background, such as web browser, music streaming, etc...
This also happens to be Apple's M.O. of how they direct users and developers into a certain direction.