Well, it is true that no third-party apps, that I'm aware of, are currently utilizing 4.0's multitasking ability, and I think a lot of folks are confusing the app-switcher list with multitasking and vice versa.
That said, the first-party/native apps already have backgrounding/multitasking/saved states/whatever-the-heck-you-wanna-call-it built in, as you can see by switching back and forth between iPod, Mail, Phone, SMS, Safari, Settings, App Store, etc.