i fixed mine. The power button is a tactile switch soldered directly to the board. theres nothing behind it to stop it from snapping off the board if it gets pushed too hard, except for some plastic "posts" to hold the part that presses the button inside. unfortunately they are only somewhat effective.
Anyway, I fixed it by aligning the tact switch with the pads on the board and soldering it back on. you will need a relatively small tip on your iron, and take care not to melt the posts I mentioned earlier. take the mobo out entirely if you need to. I protected the button from breaking off again in the future by cutting a small piece off an itunes card and sandwiching it between the back of the tact switch and the edge of the battery.