Hi JO,
Simply put it doesn't need one. While this is cool, it isn't necessarily a great thing. Here is an excerpt of a review of the display from Appleinsider.com.
"What might surprise most is the absence of a power button. Keeping with Steve Jobs' seeming fear of buttons, the new display reacts automatically. Plug a powered device (even a sleeping MacBook) into the DisplayPort jack, and it turns on; remove the cable, and it turns off. There's a certain wonderful simplicity to this, but it also limits your choices for when the display will activate. You can't turn the display off with the computer still running, and nor does the button serve to put both the display and the computer to sleep at the same time. It also makes unplugging peripherals difficult when a connected system is asleep, as there's no way to elegantly do that without first waking the computer and the display at the same time."
I think that sums it up nicely and yeah, it is a little strange.
Enjoy!