After returning from a week in London where I had no cell coverage but was constantly charging my iPhone 4 (just to be safe), I noticed that my daily iPhone usage was dropping my battery down to the 50-60% mark instead of the 65-75% mark where is had typically been.
Last weekend I let the iPhone drain all the way down, then did a full charge overnight. Since then, my remaining battery percentage has been back in the 70+% range at the end of the day.
This week I tried another experiment, going ~2 days between charges (Monday morning to Tuesday evening, then charge overnight). This has worked out fine, with about 20-40% charge left after the end of the 2 day cycle.
I’ve had similar experiences with my various MacBooks – the battery seems to hold a longer charge when you let it drain down most of the way before charging it up again. I know this is contrary to what they say about the memory effect in modern batteries, but it’s what I’ve experienced.