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In both iPhone OS 4 and Android, users should never need to quit apps manually — when the system runs low on memory, it automatically quits least-recently-used background apps to free up more.
Now, it’s true that there are task manager apps in the Android Market. But they are not necessary. The Android system doesn’t come with one and doesn’t need one. And I strongly suspect that Google’s Android team is annoyed that these task manager apps are in the market, because their existence creates the impression that they’re necessary or useful. I’ve spent a few weeks on a Nexus One, and background apps don’t slow the system down and don’t need to be quit manually.
http://daringfireball.net/2010/04/mobile_multitasking