A power-fail event (i.e. VDDOK or VCCOK low) will not cause a wakeup even if this interrupt is enabled. This will cause problems in light sleep (shutdown with display on) since software must be active to respond to a power-fail event by entering deep sleep (shutdown with display off) before the hardware can enter backup mode.
On the palmtop product, this problem was solved by creating a signal on the PCB called "PSF" that is the logical NAND of VDDOK and VCCOK. PSF is connected to Hornet's GPIO13 pin. Before entering light sleep, software enables interrupts and wakeups on a high level of GPIO13. In the event of a power fail, GPIO13 will cause a light sleep wakeup and allow the normal power-fail NMI to take effect.