There are some types of application where you need to programatically disable the keyguard. It’s also sometimes useful to temporarily do this when you are presenting a demo.  com.android.alarmclock.AlarmAlert in the Android OS source code provides and example how to do this…

    private synchronized void enableKeyguard() {
        if (mKeyguardLock != null) {
            mKeyguardLock.reenableKeyguard();
            mKeyguardLock = null;
        }
    }
    private synchronized void disableKeyguard() {
        if (mKeyguardLock == null) {
            mKeyguardLock = mKeyguardManager.newKeyguardLock(Log.LOGTAG);
            mKeyguardLock.disableKeyguard();
        }
    }

where

mKeyguardManager = (KeyguardManager) getSystemService(Context.KEYGUARD_SERVICE);

I have also come across a great class, ManageKeyguard in the source of SMSPopup. This provides an example how to disable the keyguard, show your activity and re-enable the keyguard without letting someone get past the keylock to other activities. There’s also more about this discussed on the Google groups.