Giz Explains: How Multitasking Works on a Phone
While not iPhone/iPad specific, fairly interesting reading for those who want multitasking on their portable device.
None of the four major portable device OS's reviewed offer "true" multitasking, in the form that most desktops implement it.
In a nutshell:
Android - Cooperative, system managed, backgrounding. Full access to APIs, CPU cap.
iPhone (iPad) 4.0 - Cooperative, system managed, backgrounding. Limited access to APIs.
WebOS (Palm) - System managed, backgrounding. Limited access to APIs.
Windows Phone 7 (unreleased) - System managed, "fast switching". No access to anything in dormant state.
Money quote from Android: "Remember Android's garbage battery life when it first came out? That's because there really were no restrictions on what kind of resources an app could consume in the background. "