Hi jsquareg, I'll add my observations.
I have several wireless mice, well, 2 wireless and 2 bluetooth.
The oldest of them is a Logitech. It doesn't have a power switch on the bottom, it goes into a sleep mode after a period of inactivity. In a darkened room it's easy to see the led through the translucent bottom pulsing very dimly, waiting for any movement to awaken it. It runs on 2 AA batteries, but since I don't use it very often I don't know how long the batteries will last.
The other wireless mouse is part of a wireless mouse/keyboard combo. I used it for quite some time on my home theater computer. The mouse runs on 2 AA batteries which would last about 6 months or so in daily use. This mouse also would sleep after a period of inactivity, but it requires a button press or rolling the scroll wheel to awaken. This is a very inexpensive no-name brand setup that I got at Wal-Mart.
Currently in use on my home theater computer is a bluetooth mouse/keyboard by Rocketfish from Best Buy. I got it because the wireless mouse was right at the edge of its range so was not 100% reliable, range is not a problem for the bluetooth devices. The mouse has a power switch on the bottom, but I never turn it off. It also sleeps, but simply moving it will wake it up. Two AA batteries power it for about 5 or 6 months.
Last but not least, I have a bluetooth Razer gaming mouse for my MacBook Pro. It has a very aggressive sleep mode (about 5-7 seconds of inactivity) that can be a frustrating since there is a slight delay before it wakes. I don't use it on a regular basis so I'm unsure of the battery life.
So, to echo what Seavixen32 said earlier, the operating system can't turn off the wireless mouse and keyboard. The user can, if the device is equipped with a power switch. However, even the switchless designs have a decent battery life during "average" usage. Your mileage, of course, may vary.
And by the way, I prefer the dry cell powered devices over rechargeable, when the device unexpectedly goes dead it's just so much easier to pop a couple of fresh batteries in than to wait for a recharge.