Will the iPhone's new-found multi-tasking skills be a killer feature, or just a battery killer?

I've been playing with an iPhone 4 this week and it certainly puts my old iPhone 3G to shame. It's interesting how iPhone OS updates always run like a dog on old iPhones for a while, until Apple manages to get the bugs sorted out. You'd almost think old phones were deliberately turned to mush for a while to "encourage" people to upgrade to a new iPhone. Right now my old iPhone 2G running 3.x is a lot more responsive than my iPhone 3G running 4.0.1. The plan is working, as I'm seriously considering an upgrade.

Conspiracy theories aside, one of the biggest usability changes when moving to the iPhone 4 is dealing with multi-tasking. Apple doesn't want you to worry your pretty little head about such things, just shut up and keep buying stuff from the iTunes store. But if you dare ask questions of the man behind the curtain, the answers can be a little confusing.

In day to day use, you shouldn't need to pay too much attention to multi-tasking. If you double-press the home button to call up the task manager, all is not as it seems. Even monitoring apps such as iStat can produce confusing results.

Many apps that appear to run in the background on the iPhone 4 are actually hibernated and stored in RAM, where they don't chew up CPU cycles or battery life. Call on these apps and they quickly wake from their slumber exactly as you left them - like a lazy security guard working night shift - giving the impression they were always running. It's a clever sleight of hand on Apple's part.

There is however a limited set of applications - such as background audio and sat-nav - which really do keep running in the background. If you're not careful, sat-nav apps can keep running after you're finished with them and quickly drain your battery. Worse yet, it's likely to happen when you're away from home and most need your smartphone to be up and running.

Having to worry about things like background apps is foreign to iPhone owners who are used to lording it over other smartphone users for their complicated devices. I expect iPhone 4 owners will learn the hard way that multi-tasking can be both a blessing and a curse.