Mine has started playing up, as a test I charged it to 100% this morning made sure no apps were running, loc services and iCloud off, then put it to sleep. Returned from work and found it at 68%, I've also had times when the battery meter stays at 100% for hours and hours before suddenly dropping down to the 60%-70%.
Going to be dropping into an apple store soon and see about a replacement as I can trust it won't go suddenly flat on me.