I don't know if this is common knowledge and i'm just out of the loop, but i have already done this twice and its the same result. My battery gets down below 40%, i turn the phone off for a couple of minutes, start it up and there's an extra 20%-30% of charge that actually lasts.
I took some pictures of JuicePlotter and it shows the charge before and after reboot and you can clearly tell the sudden spike. The green glow at the bottom of the graph indicates charging, and you can clearly see i wasn't.
Anyway, thought it was pretty interesting.