Basically people noticed with the new iPad and its humongous battery that it keeps charging past 100%.
Apple came forward and said this was a 'feature'. Now, we've heard this before, it's the biggest cover up for a bug in the history of tech...
Only this time Apple's explanation actually sounds pretty good to me. The charge level reads 100%, so as far as the user is concerned they have all the juice they're supposed to have. But behind the scenes, iOS continually charges and discharges the battery a little, keeping it in tip-top condition. So no matter how long an iOS device is hooked up for the mains, the battery should stay reasonably healthy and the device should be ready to go with '100% battery' when unplugged.
That this was noticed on the new iPad is due to the bigger battery making the effect more noticeable if you're monitoring the devices power draw.
This, combined with Apple's rather aggressive shutdown of the phone when the battery is close to being (but not entirely) exhausted, is a great way of keeping batteries healthy and therefore devices working longer IMHO. Perhaps it's technologies like these which mean so few of us ever need to get the batteries in our iDevices replaced (and just as well, given that they're sealed in).