For the most part I think applications obeying the "launch fast/quit fast" protocol implied by the lack of background tasks will do just fine. There's plenty you can do.
I can think of one major stumbling block though, and that's tasks that you can't just quit and suspend. Like, say uploading a file. Unlike mail on the phone, where you can send a mail, lock the phone, and hear the satisfying swoosh that lets you know your mail was delivered, if you lock, get a call, or accidentally hit home, good bye file upload. I can see ways to handle it sort of with UI, but noting jumps out as being as good as Mail's solution.
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