(void)OSSetPowerCallback( PowerCallback );


static void PowerCallback()
{
    s_shutdown = TRUE;
}


static void CheckSystemButton()
{
    if( s_shutdown )
    {
        OSReport("Power button was pushed.\n");
        OSShutdownSystem();
    }
    if (OSGetResetButtonState())
    {
        OSReport("Reset button was pushed.\n");
        OSRebootSystem();
    }
}

In main loop:
CheckSystemButton();




Other:
static void ResetCallback();
static void PowerCallback();
static bool reset_called,power_called;

    OSSetResetCallback(ResetCallback);
    OSSetPowerCallback(PowerCallback);

    reset_called = false;
    power_called = false;

static void ResetCallback()
{
    reset_called = true;
}

static void PowerCallback()
{
    power_called = true;
}


      /* Process executed when returning from the HOME Menu */
                switch( HBMGetSelectBtnNum() )
                {
                case HBM_SELECT_HOMEBTN:
                    break;
                /* Move to the Wii Menu */
                case HBM_SELECT_BTN1:
                    OSReport( "Return to WiiMenu.\n" );
                    OSReturnToMenu();
                    break;
                /* Reset */
                case HBM_SELECT_BTN2:
                    OSReport( "Reset.\n" );
                    OSRestart( 0 );
                    break;
                case 3:
                    
                    break;
                default:
                    break;
                }


/* Process executed when the RESET or Power Button is pressed */
        if(reset_called)
        {
            /*When other than the HOME Menu, go straight on to reset */
            if( homeBtnSwitch == OFF )
            {
                OSRestart(0);
            }
            /* If the HOME Menu is running, reset after black-out */
            else
            {
                HBMStartBlackOut();
            }
            reset_called = false;
        }
        
        if(power_called)
        {
            OSReturnToMenu();
        }


Need to handle/hook:  
-OSReturnToMenu(); //Returns to the system menu
-OSRestart(0); //Restarts the application.
-OSRebootSystem(); //Reboots the entire system.
-OSShutdownSystem(); //Shuts down the system

void OSRebootSystem         ( void );
void OSShutdownSystem       ( void );
void OSRestart              ( u32 resetCode );
void OSReturnToMenu         ( void );
?? void OSReturnToDataManager  ( void ); ??


OSResetCallback OldResetCallback;
OSPowerCallback OldPowerCallback;

  /* Set up callback functions */
    OldResetCallback = OSSetResetCallback(ResetCallback);
    OldPowerCallback = OSSetPowerCallback(PowerCallback);
    reset_called = FALSE;
    power_called = FALSE;


OSResetSW.h
typedef void (*OSResetCallback)(void);
typedef void (*OSPowerCallback)(void);

BOOL            OSGetResetButtonState( void ); //Checks the current state of the RESET Button.

OSResetCallback OSSetResetCallback   ( OSResetCallback callback );
OSPowerCallback OSSetPowerCallback   ( OSPowerCallback callback );


typedef BOOL (* OSShutdownFunction )( BOOL final, u32 event );
typedef struct OSShutdownFunctionInfo OSShutdownFunctionInfo;

struct OSShutdownFunctionInfo
{
    // public
    OSShutdownFunction      func;
    u32                     priority;

    // private
    OSShutdownFunctionInfo* next;
    OSShutdownFunctionInfo* prev;
};

void OSRegisterShutdownFunction     ( OSShutdownFunctionInfo* info );
void OSUnregisterShutdownFunction   ( OSShutdownFunctionInfo* info );


Flow:
Disable interrupts when doing critical processing:
BOOL interruptLevel = OSDisableInterrupts();
OSRestoreInterrupts(interruptLevel);