Yes, just to summarize what the P89C66X low power requirements (for power down...NOT idle mode):

To properly terminate power down mode, the external interrupt must be held long enough for the oscillator to restart and stabilize (normally less than 10ms).

I too do not think the keypad could be pressed and released faster than this minimum requirement. However many other sources of power wake up do not meet this requirement. I believe Kais one shot would be perfect for this.

thank you