I had my wireless keyboard for several years before this happened. Literally, the keyboard was fine one day, the next I had to replace the batteries. From then on, I replaced the batteries EVERY morning. It was simply an addition to the morning routine: wake up computer, replace batteries, get coffee, go to work.
Like you jwrusa, I checked the batteries in my charger, my mouse, or some other battery eater. They were dead as doornails. I came to the community and looked up what others had to say. No, I hadn't updated or upgraded anything - at least not immediately before this occurance. In fact, the updates and upgrades had ALL been performed, and the keyboard never wavered; it worked just fine after each of them.
The only thing that had changed in the overall scheme of things was that I had added an Air to my desk. As odd and stupid as it sounds, every time I add a new computer, one of my older ones pitches a hissy. But, for the Air, I did get AppleCare, because I take it with me to all kinds of places.
So after looking through this community and a couple others, I decided that while the end result was the same - dead batteries - the symptoms were different. I did what I should have done months before: I called Apple. Apple told me the keyboard was covered under my AppleCare plan for the Air. They gave me a couple choices:
1) Apple would send me a box, into which I would put the keyboard, they would pay to ship old keyboard to them and ship me a new keyboard. At no cost.
2) I could drive the keyboard to an Apple Store and they would give me a new keyboard. At no cost.
My nearest store is about 110 miles away. Any excuse to go to the Apple Store is good by me. So I made an appointment and drove down to the store. I got there early enough to browse a bit, met my person, handed over the keyboard - with batteries so he could check them. He played with it and, yup, batteries were dead. He went into the back room and came out with a new keyboard AND a new set of batteries!
No muss, no fuss, just deal with the problem. Which is why I have had Apples since the Apple II series.