A lot of people made a big thing about how the iPhone didn't have multi-tasking. But personally, I never saw any of the reasons as very compelling. Given limited cpu power and memory and a tiny screen, it made perfect sense to single task.
Sure, there are probably a few reasonable uses of multi-tasking. But there are way more potential abuses.
I look at my Windows machine, which is continually thrashing away when I'm not doing anything. Every piece of software I install wants to run stuff in the background. Just to check for updates once a week they want to have a process running constantly. That's not a benefit, that's abuse.
Two days after I installed iOS 4 with its long awaited multi-tasking, I went to use my iPhone in the morning and the battery was totally dead - it wouldn't even turn on. It had plenty of power the evening before. I can't be 100% sure of the culprit, but I assume it must have been some application left running by the new multi-tasking. I suspect it was a mapping app that I'd been playing with, probably running the GPS constantly or something like that.
On the other hand, according to Apple, most apps do not actually run in the background, they just get suspended and resumed. If that's really true in all cases, then I'm not sure what drained my battery overnight.
So now I find myself regularly "killing" all the active apps because I'm paranoid about this kind of scenario. Great, a "feature" has imposed a large manual overhead on me.
Hopefully the situation will improve once apps are updated to work better with the multi-tasking. But there will always be poorly behaving apps.
What I find strange is that Apple went from not allowing multi-tasking, to making it the default, with no way to switch it off. It seems like an option that you had to explicitly turn on (or at least some way to disable it) would have still silenced the critics, but wouldn't have imposed the cost on those of us that didn't want it.