In 2009, iPhone multitasking is a bit like apps were in the early jailbreak days. That is to say painfully, clearly possible, but simply not allowed. These jailbreak apps show how it could—and should—be done.
What you're seeing here is really the combination of two apps, standby jailbreak justifier and essential iPhone app Backgrounder, which lets your designate any app to run in the background, and new task switcher interface called Multifl0w. (There are other, more basic task switchers already, the most widespread being Kirikae) The new combo feels like magic: It's a little bit Android, a lotta bit Pre, and more importantly, an obvious improvement, at least on the speedier 3GS.
Granted, anyone who's used background knows that for the sake of your battery, you have to be careful how many apps you open, and how many you leave running. Honestly though? Every other smartphone manufacturer trusts their users to mind their own damn processes, which seems to work out pretty well. So, uh, when will we get this by default? OS 4.0? 5.0? Shut up, blogger?
You can give it a try now in jailbreak app manager Rock, and Cydia's on its way. Sadly, it's only free on a trial basis, after which it'll cost your five dollars. Backgrounder and Kirikae, though? They're still free, in all sense of the word. [MultiFl0w--Thanks, William!]